A double-blind investigation was undertaken to compare the efficacy of nalbuphine and fentanyl in the prevention of pain after day case termination of pregnancy. Forty patients were allocated randomly to receive nalbuphine 0.25 mg/kg or fentanyl 1.5 micrograms/kg immediately before induction of anaesthesia. The patients completed scores for pain and nausea, and performed a reaction time test to assess recovery. An observer assessed patient appearance at 1, 2 and 4 hours postoperatively. Patients who received nalbuphine had significantly lower pain scores at 1 hour (p less than 0.01) and 2 hours (p less than 0.05) and required significantly (p less than 0.05) less postoperative analgesia. No significant differences were found between the groups for incidence of nausea or for observer assessment of appearance. There was some evidence of psychomotor impairment at 2 hours in the nalbuphine group. Freedom from Controlled Drug Act regulations and improved analgesia with nalbuphine, render it more satisfactory for day case surgery than the more commonly used fentanyl.