Anaesthesia, postoperative analgesia and the incidence of adverse effects after intrathecal pethidine hydrochloride 0.50 mg.kg-1 and 0.75 mg.kg-1 were assessed and compared with a conventional technique using isobaric bupivacaine 13.75 mg in patients undergoing transurethral resection of the prostate gland. Sensory and motor block were significantly shorter with both pethidine regimens than with bupivacaine (p < 0.001). Although sensory and motor block were shorter after pethidine 0.50 mg.kg-1 than after pethidine 0.75 mg.kg-1 the difference in duration was clinically insignificant. The incidence of incomplete motor block was significantly greater with pethidine 0.50 mg.kg-1 than with bupivacaine 13.75 mg.kg-1 (p < 0.01). Visual analogue pain scores recorded after the operation were low and were similar in the pethidine groups to those obtained with bupivacaine alone. Mean arterial blood pressure was significantly lower in both pethidine groups compared with the bupivacaine group between 30 and 240 min after intrathecal injection (p < 0.001). However, the within-group reductions in blood pressure were within clinically acceptable limits. The incidences of nausea and emesis were low and emesis occurred in patients in the bupivacaine group only (p < 0.03). Pruritus was seen only in patients receiving pethidine. Intra-operative sedation occurred more often in patients receiving both pethidine 0.50 mg.kg-1 and 0.75 mg.kg-1 compared with patients receiving bupivacaine (p < 0.04). Both pethidine regimens provided acceptable anaesthesia and there were no significant differences between the two regimens in quality of intra-operative anaesthesia, incidence of adverse events or postoperative analgesia.