Sildenafil is a selective inhibitor of phosphodiesterase type 5 (PDE5), which has been shown to be a clinically effective treatment for erectile dysfunction. Its action results from increased levels of cyclic guanosine monophosphate (cGMP), which is normally degraded by PDE5. This cyclic nucleotide is a second messenger for nitric oxide, which is involved in the regulation of numerous functions, including vascular smooth muscle tone. In an attempt to better predict the effects of sildenafil on cardiovascular function, the distribution of PDE activity was determined with anti-PDE1 and anti-PDE5 antibodies in the human cardiac ventricle and saphenous vein, and in vitro studies were performed on the isolated human cardiac ventricle, corpus cavernosum, saphenous vein, and mesenteric artery as well as on rabbit aorta, dog coronary artery, dog trabecular tissue, and rabbit and human platelets. The major PDE activity in the human cardiac ventricle was shown to be calcium/calmodulin-dependent PDE1, but there was no detectable level of PDE5. In contrast, the human saphenous vein contained PDEs 1, 4, and 5, and the human mesenteric artery contained PDEs 1, 2, 3, 4, and 5. The distribution of PDE5 in the cardiovascular system is consistent with the observed pharmacodynamic and clinical effects of sildenafil. Sildenafil, unlike milrinone, a selective PDE3 inhibitor, had no effect on the isolated trabeculae carneae; this is consistent with the lack of PDE5 expression in cardiac myocytes. Sildenafil selectively increased cGMP levels in coronary vascular smooth muscle tissue but produced no change in cyclic adenosine monophosphate (cAMP) levels, which is consistent with the drug's selectivity for PDE5. In phenylephrine-contracted isolated rabbit aortic rings, sildenafil enhanced the relaxation induced by the nitric oxide donor glyceryl trinitrate, suggesting that sildenafil may potentiate the hypotensive effects of nitric oxide donor agents on the vasculature, an effect that has been observed clinically. Human platelets were found to contain PDE5, which was inhibited by 50% (IC50) by sildenafil at a concentration of 6.3 nM, consistent with the IC50 value in the corpus cavernosum. Sildenafil alone had no direct effect on platelet function, but it potentiated the in vitro antiaggregatory activity of sodium nitroprusside on rabbit and human platelets. The pharmacodynamic and adverse event profiles observed in clinical trials with sildenafil are consistent with the in vitro profile of the tissue distribution of PDE5 and its known mechanism of action as a selective inhibitor of PDE5.