We have examined the effects of extradural clonidine 150 micrograms or morphine 4 mg on postoperative pain, stress responses, cardiopulmonary function and motor and sensory block in a double-blind, randomized study in 20 patients undergoing hysterectomy with general anaesthesia. Observations were made for 6 h after each patient's first request for analgesia. Clonidine provided greater pain relief than morphine only for the first 2 h of observation (P less than 0.001). Plasma cortisol concentrations decreased to a greater extent (P less than 0.05) with morphine, while plasma glucose concentration increased by a similar extent in both groups. After clonidine, mean arterial pressure decreased from 100 (SEM 3) mm Hg to 70 (3) mm Hg (P less than 0.05), but there was no change after morphine. There were no significant changes in heart rate, pulmonary function (FEV1), motor function or sensory analgesia to touch, temperature and pinprick in both groups. Additional systemic opioids were required by five and six patients in the clonidine and morphine groups, respectively.