A comparison of alfentanil and fentanyl was made with special reference to their postoperative effects. The study was performed double-blind in one hundred patients of either sex, undergoing elective surgery for hernia nuclei pulposi. All patients received thiopental, pancuronium, droperidol and 1-2 ml of a randomly selected ampoule, containing either 0.5 mg per ml alfentanil or 0.05 mg per ml fentanyl, for induction of anaesthesia. After positioning the patient, a 3-4 ml bolus dose of the analgesic was administered and occasional increments were given thereafter. There were significantly fewer responses to intubation in the 'alfentanil patients' than in the 'fentanyl patients'. Extubation time, although of little clinical importance, was significantly shorter in the alfentanil group. 12 Patients who were treated with alfentanil and 10 patients who were treated with fentanyl required the administration of an analgesic after mean time intervals of 65 and 64 min, respectively. Recovery of consciousness was similar in the two groups, but alertness 45 min after completion of the operation was significantly better in the alfentanil group: 52% were fully awake, as compared with 30% after administration of fentanyl.