Six races of Peromyscus, from a variety of habitats, have been studied with respect to factors involved in adaptation to environmental temperature. The central theme of the study was to assess the extent to which metabolic rate is involved in the processes of distribution and speciation in this genus. A unique, electrolytic respirometer for the accurate measurement of oxygen consumption was constructed and reported in the literature. With this apparatus, the metabolic rate characteristics of the six races were measured over the temperature range 0° to 35°C. After acclimation to standardized laboratory conditions, critical temperatures and metabolic responses to temperatures below thermoneutrality were primarily related to body size. They show, therefore, no evidence of racial metabolic rate adaptation or significant insulative differences. Body weight per se is not correlated with the climate of the respective habitats. A single equation which predicts the metabolic rate of these races at any temperature between 0° and 27°C, from a knowledge of body weight and body temperature, is derived. When considered as a single group, the basal oxygen consumption of all races varied with body weight 0.69 and was insignificantly different from the accepted interspeciesapproximation. The basal metabolic rates of each race showed no temperature-adaptive differences, especially when considered in relation to body composition. The body composition in terms of water, fat and protein was determined for all individuals. The importance of considering body composition, especially fatness, in comparative studies of metabolic rate is emphasized. It is concluded that metabolic rate is inadaptive to climate in these races of Peromyscus and consequently has played no important role in their distribution and speciation. It is shown that the major temperature-adaptive feature of these small mammals is the use of a suitable microclimate. Measurements of the ambient temperatures prevailing in the microhabitats of the six races during winter and summer are presented. These data indicate that there is no significant differential selective pressure for temperature adaptation among the six races, and are complementary to the finding that metabolic rate is inadaptive to gross climate in Peromyscus. / Science, Faculty of / Zoology, Department of / Graduate