The sedative and anesthetic effects of ethanol and propofol when these drugs are coadministered are not known. Accordingly, we investigated the nature of the pharmacological interaction between ethanol and propofol during hypnosis and anesthesia in the mouse. Propofol, ethanol, and mixtures of the two were administered through the tail vein in male CD-1 mice (n = 162). The loss of righting response occurring 10 s after injection and persisting at least 10 s thereafter was defined as hypnosis, and lack of a motor response to tail clamping 60 s after injection was defined as anesthesia. The 50% effective dose (ED50) values for the hypnotic and anesthetic actions of the drugs were determined with quantal dose-response curves, using probit analysis. The pharmacological interactions were identified by the locations of ED50 values on their corresponding hypnosis and anesthesia isoboles. For each drug alone, the hypnotic and anesthetic ED50 values with 0.95 confidence intervals were 16.70 (11.98, 23.20) mg/kg and 25.02 (20.27, 31.29) mg/kg for propofol and 0.88 (0.81, 0.95) g/kg and 1.80 (1.45, 2.23) g/kg for ethanol, respectively. For the drugs in combination, the ED50 values for hypnosis with 0.95 confidence intervals were 6.98 (6.50, 7.49) mg/kg propofol with 0.61 (0.57, 0.66) g/kg ethanol, and for anesthesia were 10.55 (9.76, 11.42) mg/kg propofol with 0.93 (0.86, 1.05) g/kg ethanol, respectively. When plotted isobolographically, we found these combinations to be behaviorally additive both for hypnosis and anesthesia. Although a finding of synergism would have excluded the possibility of an identical mechanism of action for the drugs, elucidation of the molecular basis of the additivity must await further studies.