This study was undertaken to develop an anesthetized dog heatstroke model. Forty-six animals were anesthetized with pentobarbital sodium (25 mg/kg) intravenously, and maintained at an ambient temperature of (42-46 degrees C) with a water-heated blanket over 2.5-3.0 h until rectal temperatures rose to 43.0-44.5 degrees C. Animals then cooled passively until death occurred or until 18 h elapsed, and were prepared for autopsy. Liver, kidney, and brain temperature, mean weighted skin temperature, mean weighted surface heat loss, and metabolic rates were obtained. There were no significant differences between liver, kidney, brain, and rectal temperatures during the heating and cooling periods. Cardiac output rose to 127% of initial value, and dropped rapidly to zero at 43.4 degrees C rectal temperature. The rapid decline was accompanied by a doubling of heart rate and a rapid drop in blood pressure and respiratory rate. Cheyne-Stokes respiration and apnea preceded bradycardia followed by asystole or ventricular fibrillation. Certain serum constituents demonstrated modest elevations suggestive of widespread tissue damage. Autopsy did not reveal a clear pattern of heat injury, with the exception of consistent congestion of the major organs and karyorrhexis of lymphocytes. These data are in agreement with similar data from human heatstroke victims and other heatstroke modeling in dogs, and support the concept that the anesthetized dog can in many respects provide an adequate model for human heatstroke.