The effect of epidural anaesthesia with bupivacaine 0.5% on platelet aggregation was studied in seven patients undergoing transurethral resection of the prostate. Peak plasma concentrations of bupivacaine 470 +/- 270 ng ml-1 occurred at 30 min after administration. At that time there were no significant changes in platelet aggregation. However, the maximum rate of the primary- and secondary-aggregation velocities induced by 1.0 microM ADP were significantly decreased at 1 h and 3 h after bupivacaine administration. The maximum percentage ADP-induced platelet aggregation was also decreased significantly at 1 h and 3 h. The minimum concentration of ADP required to induce secondary-phase platelet aggregation was significantly increased at 1 h but not at 3 h. There was a significant correlation between bupivacaine concentrations and all platelet aggregation parameters except the maximum ADP-induced aggregation. Platelet inhibition occurred at plasma bupivacaine concentrations that were considerably lower than those needed to produce similar inhibition in vitro.