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Glucocorticoids are steroid hormones that affect a variety of physiological and pathological processes both throughout development and in adult life. During mammalian fetal growth, the late gestation rise in fetal glucocorticoid levels is essential for the maturation of tissues and organs in preparation for birth. In humans, glucocorticoids are routinely administered to women threatened by a preterm labour to accelerate fetal lung maturation and prevent neonatal respiratory distress and mice lacking glucocorticoid receptor (GR-/- mice) die neonatally as they are unable to inflate their lungs due to severe pulmonary immaturity. Apart from their importance for proper lung maturation, the physiological role of glucocorticoids in the development of other organs and tissues is not well known. However, prenatal exposure to excess glucocorticoids was shown to elicit detrimental “programming” effects, raising the susceptibility to adult diseases such as hypertension, obesity and metabolic disturbances in both humans and animal models. I therefore used global and conditional GR knock out mouse models to investigate the role and importance of adequate glucocorticoid signalling in fetal heart development and maturation. I further confirmed the direct effects of glucocorticoids on the cardiomyocyte structure and function in an in vitro setting. GR-/- fetuses are under-represented in late gestation (>50% of the number of GR+/+ littermates) but are present in the expected mendelian ratio at E14.5. At E17.5, GR-/- fetuses show edema (increased fluid accumulation and body sodium content). Excess extracellular fluid accumulation could be a result of a congenital heart failure. During development, corticosterone levels sharply increase within the fetal hearts at E15.5-E16.5, coincident with nuclear translocation of GR. Consistent with activation of GR only after this time, the phenotypic consequences of GR deficiency can be seen after E16.5 and not before. At E17.5, hearts of GR-/- fetuses are smaller than in GR+/+ but display no structural abnormalities. Cardiac function however is severely impaired, with left ventricular systolic and diastolic performance inferior in GR-/- fetuses compared to their wild-type littermates. Microscopically, at E17.5, the structure of the cardiac muscle and individual cardiomyocytes are affected by the lack of GR. The normal outer muscle layer, with characteristic rod-shaped, aligned cardiomyocytes is not discernable in the GR-/- heart. Within the cardiomyocytes, myofibrils are short, undefined and randomly scattered within the cell. Lack of the maturational progression in the GR-/- hearts at E17.5 is evident in the pattern of gene expression. GR-/- fetuses do not display the normal gestational changes between E14.5 and E17.5 that are seen in control mice, including in genes involved in the maturation of cardiac structure (eg myosin heavy chain-α, MyHC-α), function (atrial natriuretic peptide, ANP), energy metabolism (eg hexokinase-1, PPARγ coactivator-1α, PGC-1α) and calcium handling (ryanodine receptor, RyR; sarcoplasmic reticulum Ca2+-ATPase, SERCA2a). However, there are no genotype or gestational alterations in mRNA encoding the mineralocorticoid receptor, which is also a receptor for glucocorticoids in the heart. The normal gestational changes in the levels of modified histone H3 associated with the promoters of some of the genes (MyHC-α, ANP, PGC-1α) are not seen in hearts of GR-/- fetuses. This cardiac phenotype was not secondary to adrenal catecholamine insufficiency reported in other GR-/- models, as peripheral tissue levels of adrenaline were not different between genotypes. In order to test the hypothesis that the effects of glucocorticoids on the heart are mediated via GR in cardiomyocytes and to further elucidate the direct effects of GR deficiency specifically within the heart, mice with conditional deletion of GR selectively in cardiac and vascular smooth muscle cells were generated ("SMGRKO" mice). These show ~65% reduction in cardiac GR mRNA and protein levels. Circulating levels of corticosterone do not differ between genotypes at E17.5. SMGRKO fetuses at E17.5 display a phenotype strikingly similar to that of global GR-/-, namely edema, impaired cardiac function, impaired cellular architecture within the ventricle and alterations in the gene expression, implying that the GR-deficient phenotype is largely due to the direct actions of GR within the heart and not secondary to effects on other systems (eg kidney or liver). In order to investigate the pathways by which GR stimulates cardiomyocyte maturation, an in vitro model of murine primary fetal (E15.5-E16.5) cardiomyocytes was developed. Cultures contain >98% of troponin Tpositive cells which beat spontaneously. Treatment of cardiomyocytes with either synthetic (dexamethasone) or physiological (corticosterone) glucocorticoid induces time- and dose-dependent changes in gene expression, consistent with glucocorticoid-dependent changes seen in vivo in the late gestation heart. The effects of glucocorticoids on gene expression were abolished by either siRNA mediated knock-down of GR or RU486 antagonism of GR, but were unaffected by a mineralocorticoid receptor (MR) antagonist. Moreover, cycloheximide pretreatment (to block protein synthesis) suggested PGC-1α as a direct genomic target of GR. RNAseq transcriptome analysis performed on cardiomyocytes treated with dexamethasone and cycloheximide for 2h identified >600 genes as possible rapid and direct glucocorticoid response targets. Among them are genes involved in energy metabolism, calcium handling and sarcomere assembly. Glucocorticoid treatment of fetal cardiomyocytes also induces striking structural changes – formation of stress troponin T-associated actin fibers and sarcomere assembly. Spontaneous contractile activity is improved by glucocorticoid treatment, with a decrease in both contraction and relaxation time (without a change in frequency) and an improvement in the relaxation kinetics. In summary, glucocorticoid signalling in cardiomyocytes is required for the functional, structural and transcriptional maturation of the fetal heart in late gestation in vivo. Glucocorticoid treatment of primary murine fetal cardiomyocytes replicated the contractile, transcriptional and structural changes seen in vivo and was dependent on GR. Thus, GR is essential in cardiomyocytes for the structural and biochemical changes that underlie the maturation of heart function around the time of birth and an inadequate glucocorticoid environment could potentially lead to detrimental and permanent changes in postnatal cardiac function. Since prenatal glucocorticoids are routinely used clinically, it is important to consider any possible effects they might have on the heart development and its function later in life.