The effect of supplementing postoperative patient-controlled analgesia (PCA) with a mandatory fixed-rate infusion was studied using alfentanil. Patients were assigned by lottery to receive alfentanil PCA with a bolus of 200 or 300 mcg or alfentanil PCA with a bolus of 100 or 200 mcg plus an infusion of 900 mcg/hr, such that ten patients received each treatment. Seven patients receiving PCA only, compared to two in the groups receiving infusion, were withdrawn due to inadequate pain relief (0.1 > P > 0.05). One patient receiving PCA only was withdrawn because of a low respiratory rate, whereas three patients receiving PCA plus infusion developed respiratory depression. Patients prescribed PCA plus infusion received significantly more alfentanil but did not describe less pain than patients prescribed PCA only. The blood alfentanil concentration immediately prior to demands was significantly higher in the PCA plus infusion groups. These results suggest that during PCA therapy the drug prescription influences the blood concentration associated with satisfactory analgesia.