The efficacy of intravenous (iv) and epidural infusions of alfentanil for postoperative pain relief was investigated in 24 patients (ASA physical status 1-2) who were scheduled for abdominal hysterectomy. The patients were allocated randomly to receive either epidural or iv alfentanil. In both groups, a loading dose of 15 micrograms.kg-1 was administered, followed by a constant rate infusion of 18 micrograms.kg-1.h-1 alfentanil for 20 h. Both routes provided similar degrees of analgesia; however, analgesia occurred earlier in the intravenously treated group (P less than 0.03). Mean plasma alfentanil concentrations (Cps) varied between 42 and 82 ng.ml-1 in the iv group and 23 and 68 ng.ml-1 in the epidural group, with higher concentrations in the iv group for the first 60 min only (P less than 0.01). Cps increased with infusion time, suggesting accumulation of alfentanil. After infusion ended, pain recurred at the same time in both groups, whereas the alfentanil Cps still were greater than 45 ng/ml. Postoperative epinephrine concentrations decreased after 60 min of infusion (P less than 0.02), whereas, after 6 h, cortisol levels decreased to preoperative values. Norepinephrine concentrations decreased only slightly. The only clinically meaningful effect on vital signs that occurred was an abrupt reduction of respiratory rate after the iv loading dose. PaCO2 increased to the same extent in both groups during the first 15 min only. The incidence of opioid-related side effects was similar in both groups. These results suggest that the iv and epidural routes were equally effective for providing postoperative pain control and controlling the postoperative response to surgical stress.