The sedative effects of midazolam, morphine, and their combination were studied in ASA physical status I and II patients. The visual analog method was used to determine sedative effect. A self-rated score of 50 mm or more on a 100-mm line was regarded as a positive response to the treatment. The dose-response curves for midazolam, morphine, and their combination (each in a group of 30 patients) were determined by probit procedure and compared with isobolographic and algebraic (fractional) analyses. Interactions between midazolam and morphine when used to produce sedation represent summation, not synergism.