To obtain evidence for an ethanol mediated disruption of hemostasis we compared the effects of acute and chronic ethanol ingestion on platelet reactivity. Since ADP may be important for hemostasis in vivo it was used to induce platelet aggregation in platelet-rich plasma. Thromboxane B2 (TXB2) formed during the aggregation was measured by radioimmunoassay. Ethanol (1.5 g/kg) given to 8 healthy non-alcoholic male volunteers increased platelet reactivity to ADP and the associated TXB2 formation rose from 289 +/- 60 (mean +/- SE) to 984 +/- 263 fmol/10(7) platelets (p less than 0.025). The effects lasted for as long as ethanol was present in blood. In 13 non-cirrhotic male alcoholics the withdrawal of ethanol caused a 4-fold increase in TXB2 formation within one week but the basal levels before ethanol withdrawal were the same as in controls. These findings are discussed in relation to the higher risk of brain infarction seen in alcoholics and even associated with binge drinking. Further studies are needed to establish the effects of ethanol on prostacyclin formation.