Twenty consecutive patients undergoing elective transurethral resection of the prostate were allocated randomly to one of two groups. Group I (n = 10) received lumbar extradural analgesia with 0.5% bupivacaine. Group II (n = 10) received general anaesthesia with spontaneous respiration, using 60% nitrous oxide and 1-2% halothane in oxygen. There was a significant inhibitory effect on platelet aggregation in the extradural group. No such effect was observed in the general anaesthesia group. Measured indices of coagulation and fibrinolysis showed no abnormalities compared with control in either group except for a significant decrease in alpha 2-antiplasmin during surgery in group II. These results suggest that the possible thromboprophylatic effect of extradural analgesia with bupivacaine may result from an inhibitory effect on platelet aggregation which is in addition to the increase in lower limb blood flow.