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Background. Hydrogen sulphide, H2S, is the third endogenous gas with putative cardiovascular properties, after nitric oxide and carbon monoxide. H2S is a vasorelaxant, while H2S deficiency is implicated in the pathogenesis of hypertension and atherosclerosis. Cystathionine β-synthase (CBS), cystathionine γ-lyase (CSE) and 3-mercaptopyruvate sulphurtransferase (MPS) catalyze H2S formation, with different relative efficiencies. Chronic kidney disease (CKD) is characterized by elevation of both plasma homocysteine and cysteine, which are substrates of these enzymes, and by a high prevalence of hypertension and cardiovascular mortality, particularly in the haemodialysis stage. It is possible that the H2S-generating pathways are altered as well in this patient population. Methods. Plasma H2S levels were measured with a common spectrophotometric method. This method detects various forms of H2S, protein-bound and non-protein-bound. Blood sulphaemoglobin, a marker of chronic exposure to H2S, was also measured, as well as related sulphur amino acids, vitamins and transcriptional levels of relevant genes, in haemodialysis patients and compared to healthy controls. Results. Applying the above-mentioned methodology, H2S levels were found to be decreased in patients. Sulphaemoglobin levels were significantly lower as well. Plasma homocysteine and cysteine were significantly higher; vitamin B6, a cofactor in H2S biosynthesis, was not different. H2S correlated negatively with cysteine levels. CSE expression was significantly downregulated in haemodialysis patients. Conclusions. Transcriptional deregulation of genes encoding for H2S-producing enzymes is present in uraemia. Although the specificity of the method employed for H2S detection is low, the finding that H2S is decreased is complemented by the lower sulphhaemoglobin levels. Potential implications of this study relate to the pathogenesis of the uraemic syndromemanifestations, such as hypertension and atherosclerosis. Background. Hydrogen sulphide, H2S, is the third endogenous gas with putative cardiovascular properties, after nitric oxide and carbon monoxide. H2S is a vasorelaxant, while H2S deficiency is implicated in the pathogenesis of hypertension and atherosclerosis. Cystathionine beta-synthase (CBS), cystathionine gamma-lyase (CSE) and 3-mercaptopyruvate sulphurtransferase (MPS) catalyze H2S formation, with different relative efficiencies. Chronic kidney disease (CKD) is characterized by elevation of both plasma homocysteine and cysteine, which are substrates of these enzymes, and by a high prevalence of hypertension and cardiovascular mortality, particularly in the haemodialysis stage. It is possible that the H2S-generating pathways are altered as well in this patient population.Methods. Plasma H2S levels were measured with a common spectrophotometric method. This method detects various forms of H2S, protein-bound and non-protein-bound. Blood sulphaemoglobin, a marker of chronic exposure to H2S, was also measured, as well as related sulphur amino acids, vitamins and transcriptional levels of relevant genes, in haemodialysis patients and compared to healthy controls.Results. Applying the above-mentioned methodology, H2S levels were found to be decreased in patients. Sulphaemoglobin levels were significantly lower as well. Plasma homocysteine and cysteine were significantly higher; vitamin B-6, a cofactor in H2S biosynthesis, was not different. H2S correlated negatively with cysteine levels. CSE expression was significantly downregulated in haemodialysis patients.Conclusions. Transcriptional deregulation of genes encoding for H2S-producing enzymes is present in uraemia. Although the specificity of the method employed for H2S detection is low, the finding that H2S is decreased is complemented by the lower sulphhaemoglobin levels. Potential implications of this study relate to the pathogenesis of the uraemic syndrome manifestations, such as hypertension and atherosclerosis.