Alfentanil - nitrous oxide and fentanyl - nitrous oxide techniques were compared in outpatients undergoing therapeutic abortion or dilatation and curettage. Thirty patients were studied in each group. Time to awakening was similar in both groups but patients who received alfentanil responded to verbal commands one minute sooner and were alert 1.5 minutes before those who received fentanyl. At ten minutes post anaesthesia the recovery scores were the same for both groups. Patients who received alfentanil were not street-worthy earlier than those who received fentanyl. During the procedure approximately two thirds of the patients moved. This movement was vigorous in 23 per cent of the patients who received alfentanil and in 30 per cent of those given fentanyl. We conclude that: (1) a more flexible dosage schedule is required in order to prevent disturbing movement of the patient during the procedure and (2) patients who received alfentanil were not street-worthy earlier than those who were given fentanyl.