Human platelets with a high content of endogenous serotonin took up more serotonin when incubated with exogenous serotonin, than platelets with a low endogenous content of serotonin. Also, thrombin-stimulated serotonin secretion (%) was high when endogenous serotonin was high. This was not found with fresh platelets, but was found when platelets were stored as platelet concentrates for 5 and 7 days. With platelets stored for 5 or 7 days, both uptake and secretion were increased after preincubation of the platelets with an amount of exogenous serotonin that was completely taken up. Inhibition of the reuptake of serotonin by imipramine during thrombin-induced secretion increased the secretion in stored platelets. Agonists like collagen, ADP, and a prostaglandin analogue (U46619) gave only 3–7% secretion. Imipramine increased the secretion induced by U46619, but not the secretion induced by ADP or collagen. The specific 5-HT2receptor inhibitor, ketanserin, had no effect on agonist stimulated secretion, or secretion stimulated by a calcium ionophore (A23187).Both the uptake and the thrombin-induced secretion of serotonin correlated significantly with endogenous serotonin in stored platelets. In fresh platelets the uptake of serotonin correlated positively, although not significantly, with endogenous serotonin. It is speculated that endogenous serotonin may affect secretion through stimulation of the thrombin receptor, at least in stored platelets.