Ca2+ influx, Ca2+ mobilization and phospholipase C activation have been determined in human platelets stimulated by thrombin in the presence of increasing concentrations of SIN-1. Preliminary data indicate a major inhibitory effect of SIN-1 on Ca2+ influx, but also a significant inhibition of phospholipase C. However, the decrease of serotonin secretion by SIN-1 seems to be more related to phospholipase C inhibition. These data are discussed in relation to the known effects of SIN-1 on cGMP content of platelets.