The dynamics of changes in the stimulation of human platelet guanylate cyclase by some activators in aggregating platelets was studied. It was shown that ADP-induced aggregation of human platelets (donors) is accompanied by the enhancement of the intensity of guanylate cyclase activation by sodium nitroprusside, L-arginine, protoporphyrin IX and arachidonic acid and also by the increase in cGMP content. Immediately after the induction of aggregation the intensity of guanylate cyclase activation and cGMP content begin to increase. The rise reaches its maxima within several minutes, then followed by a fall to the initial level. The peaks of the enhanced capacity for guanylate cyclase activation by the above compounds coincide in time and intensity. On the basis of the proposed hypothetical scheme of cGMP action as a regulator of platelet aggregation a possible mechanism of enhancing the capacity of guanylate cyclase to be stimulated by various activators in aggregating platelets is suggested.