Two hundred and fifty children undergoing herniotomy or orchidopexy under general anaesthesia were randomly allocated to receive pre-operatively either diclofenac sodium 1 mg.kg-1 given intramuscularly or a caudal injection of bupivacaine 0.25% 1 ml.kg-1 with or without adrenaline or no analgesia. Plasma diclofenac and beta-endorphin concentrations were determined in eight and 21 patients respectively. Postoperative pain was assessed by ward nurses who were blinded to the group allocation. Comparison with the control group showed diclofenac to be an effective analgesic. Caudal bupivacaine provided more pain-free children during the early postoperative hours, but later the need for pethidine as rescue analgesic was lower among the children who had received intramuscular diclofenac. Caudal analgesia abolished the stress-induced increase in plasma beta-endorphin level which was found in the children given diclofenac and in those who served as controls. Total plasma clearance of intramuscular diclofenac sodium appears to be higher in children than in adults. A single intramuscular dose of diclofenac significantly reduces the need for an opioid analgesic in children after inguinal herniotomy or orchidopexy, and owing to its long duration of action, it offers an alternative or complementary method of pain relief to caudal analgesia.