In a placebo-controlled double blind cross-over experiment the adenosine uptake inhibitor dipyridamole (400 mg/day) did not affect ex vivo platelet aggregation induced by collagen or adenosine-diphosphate (ADP) in an electronic whole blood aggregometer (WBA). Dipyridamole was also inactive in vitro, unless red blood cell injury was deliberately enhanced, thereby increasing the level of free adenine nucleotides. Since dipyridamole also inhibits cyclic guanosine monophosphate (GMP) phosphodiesterase (PDE), we used platelet rich plasma (PRP) to study its interaction with authentic and endothelium-derived nitric oxide (NO). The latter inhibits platelets by increasing cyclic GMP. Dipyridamole (1 to 30 microM), either alone or in combination with a subthreshold concentration of prostacyclin (PGI2), was inactive. However, when combined with a subthreshold concentration of NO, dipyridamole caused a concentration-dependent platelet suppression, which became more pronounced when PGI2 was present as well. It is concluded that dipyridamole could reduce the threshold for platelet suppression by NO through inhibition of cyclic GMP PDE.