The effects of the clinically most commonly used minor tranquilizer, diazepam, on the survival time and on the mechanism of death in non-acclimated and cold-acclimated guinea pigs in severe cold exposure (-20 degrees C) were studied. Cold acclimation for 2 months increased the average survival time from 4 h to 10 h. The lowest rectal temperature at death (14.6 degrees C) was seen in the cold-acclimated animals. Diazepam at a dose of 5 or 15 mg/kg i.p. 30 min before the beginning of the exposure reduced dose-dependently the cold endurance of both cold-acclimated and non-acclimated guinea pigs. The serum glucose and free fatty acid concentrations were low in the animals with the long survival time. Histological studies of liver, kidney, and adrenal glands showed no specific changes. Exposure seemed to increase the frequency of contraction bands and to decrease focally the intensity of beta-hydroxybutyrate dehydrogenase reaction in the myocardium, which indicates a mild hypoxic lesion of the muscle cells.