In previous studies we observed modifications of cyclic adenosine monophosphate and adenylate cyclase activity in human endometrium during the menstrual cycle. In the present study our intension was to verify whether these modifications occur in isolated intact platelets. The results demonstrate that in nine normal women platelet cyclic adenosine monophosphate content varies during the menstrual cycle both in basal and in stimulated conditions (in vitro addition of forskolin). In fact, significantly higher levels of cyclic adenosine monophosphate were consistently observed during the proliferative phase. These findings provide evidence that platelet cyclic adenosine monophosphate metabolism normally varies during the menstrual cycle, which suggests a possible involvement of this system in some important clinical events.