The effect of thiopental on the induction dose-response curve for midazolam was studied in nonpremedicated ASA physical status I and II patients. As an endpoint of anesthesia, ability to open eyes on command was used. Dose-response curves for thiopental, midazolam, and their combination were determined with a probit procedure and compared with an isobolographic analysis. Interaction between midazolam and thiopental was found to be synergistic (supraadditive). The degree of midazolam-thiopental anesthetic synergism in surgical patients was close to that reported previously in rats with loss of the righting reflex as an index of anesthesia.