Acetaldehyde (ACT), both, ex vivo and in vitro did not change the ADP-induced rat platelet aggregation and the potentiating action of serotonin. The whole blood serotonin content was decreased only when ACT was used in doses of 20 and 30 mg/kg, iv and the platelet serotonin content remained unchanged. Ex vivo, ACT had no influence on the labeled serotinin uptake, whereas in experiments in vitro it inhibited the amine uptake and augmented the serotonin release from blood platelets in a dose dependent manner. The results indicate that all the changes in the platelet serotonergic mechanisms appear only after high concentrations of ACT. They may be of significant importance in the circulatory system or hemostasis only during disulfiram or calcium carbamide therapy.