1. The present study investigated the in vitro effect of dopamine on platelet responses in healthy subjects. 2. Dopamine concentrations over 5 mumol/L induced a primary aggregating response and a slight release of alpha-granule proteins, beta-thromboglobulin and platelet factor-4 in all subjects. In 25% of investigated subjects a delayed secondary aggregation was observed with dopamine concentrations over 100 mumol/L. 3. Low dopamine concentrations (5-7.5 nmol/L) increased the platelet sensitivity to other aggregating agents (adenosine diphosphate, collagen and sodium arachidonate). The effect of subaggregating concentrations of serotonin was potentiated by dopamine. 4. The effect of dopamine on platelet responses was prevented by low concentrations of alpha-adrenoceptor antagonists (phentolamine and yohimbine); antagonists of dopamine receptors (haloperidol and domperidone) were able to decrease the extent of the dopamine-induced secondary aggregating wave in the responders, but they failed to prevent the primary aggregation and the effects on platelet response to other aggregating agents. 5. The present data demonstrated that the effects of dopamine on human platelets are mainly mediated by interactions with alpha-adrenoceptors.