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In a comparative study into the mechanisms of aggregation of thrombocytes under the influence of various agents, two types of reactions were established, one realized through the system of cyclic 3′5-AMP and the other, through the system of cyclic 3′5-GMP. The former type of reaction was observed when ADP and thrombin were used as aggregation inductors; the latter was induced by serotonin. The initial link in the development of aggregation of thrombocytes induced by ATP and thrombin was an increase in the level of prostaglandin EI (by 32%), under the effect of which the activity of adenyl cyclase rose more than by 41%, and the amount of 3’5-cAMP increased by almost 180% as compared with the initial level. Unlike ADP and thrombin, stimulation of thrombocytes with serotonin brought about, as a rule, an increase in the level of prostaglandins F2 (by 60%) , which subsequently led to activation of guanyl cyclase and marked augmentation in the content of 3′5-cGMP (by 300%). The application of proteolytic enzymes with thrombolytically oriented properties caused inhibition of biosynthesis of prostaglandins and cyclic nucleotides with a clearcut disaggregating effect.