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Relaxin (RLX) is a reproductive hormone with vasodilatatory properties on several organs, including the heart. RLX‐induced vasodilatation appears to depend on the stimulation of endogenous NO production. Here, we investigate whether RLX acts on rat coronary endothelial (RCE) cells in vitroby inducing changes of NO generation and, if so, to clarify the possible mechanism of action. RCE cells were treated for 24 h with vehicle (controls) or RLX, alone or in association with inhibitors of NO synthesis or dexamethasone, which inhibits transcription of NO synthase gene. In some experiments, inactivated RLX was given in the place of authentic RLX. Expression of NO synthase isozymes II and III was analyzed by immunocytochemistry, Western blot, and RT‐PCR. NO production was evaluated by the Griess reaction for nitrite and the NO‐sensitive fluorophore DAF‐2/DA. Agonist‐induced changes of intracellular Ca2+transient were studied with the Ca2+‐sensitive fluorophore Fura 2‐AM. RLX was found to up regulate NOS II mRNA and protein and to stimulate intrinsic NO generation, likely through the activation of a dexamethasone‐sensitive transcription factor, and to decrease agonist‐induced intracellular Ca2+transient. Conversely, RLX had negligible effects on NOS III expression. By these biological effects, RLX may afford significant protection against cardiovascular disease.