The minimum alveolar anesthetic concentration (MAC) is an accepted potency measure for inhaled anesthetics. There is no generally accepted intraoperative measure of opioid potency, partly because of the difficulty in obtaining steady state biophase concentrations. We have studied the relative potency of fentanyl and alfentanil by using computer-assisted continuous infusions (CACI), in terms of reduction of isoflurane MAC. Data are presented from 79 patients in whom anesthesia was induced with thiopental and maintained with a CACI of fentanyl (set to achieve a plasma concentration of 0,1,3,6, or 8 ng/mL) or alfentanil (0, 70,210,400, or 1000 ng/mL) with a predetermined endtidal concentration of isoflurane. A determination of movement in response to skin incision was made. Without any opioid, the MAC of isoflurane was 1.25%. The maximum likelihood solution to a logistic regression model showed that fentanyl 0.5 ng/mL (95% confidence intervals [CI], 0–4.6 ng/mL) or alfentanil 28.8 ng/mL (95% CI, 0–70.9 ng/mL) resulted in a 50% isoflurane MAC reduction. In the logistic regression model, age or weight were not significant factors in the model. These results suggest that the comparative intraoperative potency ratio for alfentanil and fentanyl is 58:1, and that this methodology allows for direct intraoperative comparisons of opioid potency.