Activation of human platelets is associated with an increased level of cGMP, when total cGMP in individual samples is measured. However, by discriminating between intracellular and extracellular cGMP we were able to demonstrate that cGMP accumulates in the extracellular space only, whereas the level of intraplatelet cGMP actually decreases. Therefore, during the first minutes of platelet aggregation cGMP is released from the cell, and it thereby escapes hydrolysis by intracellular phosphodiesterases. In contrast, during direct activation of soluble guanylyl cyclase by nitrovasodilators, such as sodium nitroprusside, the newly synthesized cGMP remains mainly inside the cells. Elevation of intracellular calcium and activation of protein kinase C are likely to be involved in promoting cGMP efflux. Our results are discussed in contrast to the general hypothesis that the cGMP increase associated with platelet aggregation may represent a feedback mechanism designed to terminate early events of activating signal transduction. According to our data the apparent cGMP increase results from cGMP release from thrombocytes, rather than soluble guanylyl cyclase activation. This cGMP efflux provides a mechanism of decreasing the intracellular cGMP level upon stimulation with platelet agonists and thus favors platelet activation.