In a randomized double-blind study of 57 children (aged 6 months-6 yr), pain following lateral thoracotomy was relieved with repeated i.v. doses of morphine 100 or 50 micrograms kg-1, or buprenorphine 3.0 or 1.5 micrograms kg-1. The same drug and dosage were continued and cardioventilatory indices, pain intensity and sedation measured for an observation period of 24 h. The sums of the pain intensity differences were equal in all groups. The mean doses for the complete initial analgesia were 230 and 180 micrograms kg-1 with the larger and the smaller bolus doses of morphine and 5.8 and 3.7 micrograms kg-1 with buprenorphine, respectively. The mean duration of action of buprenorphine was slightly but not significantly longer than that of morphine. The total consumption of both morphine and buprenorphine was less when the smaller bolus doses were used. Two patients developed a degree of ventilatory depression following repeated doses of buprenorphine. Buprenorphine was equal to morphine as a postoperative analgesic.