1. Physostigmine administration has been previously shown to decrease the uptake of serotonin in human platelets. In order to test whether uptake could be inhibited as a nicotinic-cholinergic effect, the in vitro effects of nicotine on platelet 5HT uptake and efflux were examined. 2. Nicotine stimulated release of serotonin from human blood platelets, and competitively inhibited human platelet serotonin uptake in a concentration-dependent fashion at in vitro concentrations as low as 20 microM for uptake. 3. The kinetics of the nicotine effects on uptake were different from those of physostigmine. Unlike the effects of physostigmine, nicotine produced different kinetic changes, with an increase in Km and no consistent change in Vmax. 4. The efflux and inhibition of uptake paralleled that previously reported in rat brain in vitro, and was likewise similar to concentrations found previously to augment extracellular amine in other tissue preparations. However, the effects of nicotine in human platelets were not reversible by nicotinic antagonism with hexamethonium. 5. The results distinguish human platelet from rat brain with respect to nicotinic antagonism, and suggest that, at similar concentrations, nicotine may increase extracellular serotonin through differing mechanisms.