The intraoperative and postoperative effects of fentanyl and ketamine administered continuously by infusion were compared with those produced by conventional intermittent bolus administration in 100 patients. After a standardized induction with thiopental 4 mg/kg i.v., patients received either fentanyl (50 .mu.g boluses vs. 2 .mu.g/ml infusion) or ketamine (25 mg boluses vs. 1 mg/ml infusion) as i.v. adjuvants to N2O, 70% in O2. With continuous infusion, the doses of fentanyl and ketamine required were decreased 45 and 43%, respectively. The times to awakening were decreased significantly, 62 and 60%, in the fentanyl and ketamine infusion groups, respectively. Intraoperative side effects (e.g., hypoventilation, hypotension and rigidity) were less frequent in the fentanyl infusion (vs. bolus) group but did not differ in the ketamine groups. Trieger scores were consistent with a more rapid recovery in both infusion groups. Incidences of common postoperative side effects (e.g., nausea, vomiting, visual disturbances and dizziness) did not differ significantly between bolus and infusion groups. Excessive sedation was noted in 48 and 52% of patients in the fentanyl and ketamine bolus groups, respectively, compared with 4 and 8%, respectively, in the infusion groups. Discharge times were decreased by 29 and 13% in the fentanyl and ketamine infusion groups, respectively. Continuous infusion fentanyl (0.1 .mu.g .cntdot. kg-1 .cntdot. min-1) or ketamine (50 .mu.g .cntdot. kg-1 .cntdot. min-1) evidently significantly decreases the drug dosage requirement, improves intraoperative conditions and decreases recovery time compared with the traditional intermittent bolus technique.