A series of five experiments was undertaken in order to characterize the pharmacological effects of alcohol on the body temperature of the unrestrained rat. After a thermistor probe was fixed within the colon of each animal and its body temperature had stabilized, physiological saline. 2.0 g/kg or 4.0 g/kg of ethyl alcohol was given by intragastric gavage. A constant concentration of 20% was utilized with doses determined volumetrically according to the individual weight of the animal. The following observations were made: (1) At a laboratory room temperature of 22 degrees C, alcohol produced a dose-dependent decline in colonic temperature. This fall was enhanced when the rat was placed in a chamber with an ambient temperature of 8 degrees C, but reversed into a hyperthermic response when the ambient temperature of the rat was elevated to 36 degrees C. (2) If the rat was exposed for one hour to either a warm (36 degrees C) or cold (8 degrees C) ambient temperature beginning at the time of the intragastric gavage with alcohol, the body temperature of the animal correspondingly decreased or increased, respectively. The magnitude of the shift in the animal's colonic temperature depended solely on the dose of alcohol given, i.e., thermolability was enhanced to a greater extent by the r.0 g/kg dose than the 2.0 g/kg dose. Exposure of the rat to a high or low ambient temperature prior to intragastric gavage with alcohol also affected the body temperature displacement as well as the direction of change. (3) Following the 4.0 g/kg alcohol dose, the animal's colonic temperature could be controlled precisely by varying the ambient temperatures in the 26 degrees C--32 degrees C range of thermoneutrality; in fact, within limits, the rat's temperature could be held constant at any predetermined level. (4) Pretreatment of the animal with 6.0 ml of a 50% solution of glucose given subcutaneously failed to attenuate the poikilothermic effect of intragastric alcohol administered in a dose of 4.0 g/kg. (5) Comparisons of the effect on body temperature of equipotent doses of sodium pentobarbital (25 mg/kg), administered intraperitoneally, and alcohol (4.0 g/kg) given by gavage showed that either a high or low ambient temperature elevated or lowered the colonic temperature of the rat at an identical rate and magnitude. It is concluded that alcohol acts acutely as any other anesthetic agent to abolish all thermoregulatory control functions. Thus, the physiological mechanisms for the dissipation of body heat as well as those for heat production are incapacitated by the drug. Clearly, alcohol is a poikilothermic agent, and thus, the decline in body temperature observed in the animal typically kept at a laboratory room temperature of 20 degrees C--24 degrees C represents the result of a cold challenge and is not due to a postulated "hypothermic" effect of alcohol.