Body temperature and oxygen consumption were measured at various environmental temperatures in a series of captive and wild caught vampire bats, Desmodus rotundus. The response to changes in ambient temperatures was unpredictable and could not be correlated with the age or sex of the animals, their nutritional condition, or the length of their captivity. Body temperature varied greatly, but generally did not fall below 30° C when the ambient temperature was above 25° C. At lower ambient temperatures, some bats increased their metabolism and maintained a high body temperature for varying periods, while others showed little or no increase in metabolic rate, and their body temperatures declined. Below about 20° C body temperature, bats were unable to rewarm themselves without exogenous heat. Calculations indicate that vampires did not drink sufficient blood to maintain a homeothermic condition at the temperatures of temperate zone hibernacula. Bats could tolerate ambient temperatures of 33° C or more for only short periods. It is suggested that Desmodus is limited from spreading northward because of its inferior temperature regulation.