The distribution and ecology of the woodland jumping mouse, Napaeozapus insignis, were studied in the light of its behavior in the field, its physiology in the laboratory, and by comparison with other species of small rodents. Data from 36 traplines show that jumping mice have no preference for habitats near water. Shrubby ground cover appears to be the most important factor affecting their local abundance. Napaeozapus has probably been associated so often with streams because these areas favor growth of good ground cover. Woodland jumping mice were found in nonwooded areas where shrubby growth cover was found. In one nonwooded area it replaced the meadow jumping mouse, Zapus hudsonius. Removal of the population of woodland jumping mice in the fall was followed by the establishment of the meadow species the next spring. Woodland jumping mice and redback voles seldom occurred together in abundant numbers. This separation was partly the result of distinct habitat differences, but in some areas of mixed woods having ground cover, the presence of voles was accompanied by an absence of jumping mice. When jumping mice were present, redback voles were few or absent. This pattern of distribution could not be explained by differences in habitat selection or by competition. Some form of interference could be involved. Woodland jumping mice show a tendency to be more active on colder nights. This behavior contrasts with observations on deer mice by other workers who state that these mice are more active on warm, cloudy nights. The high population densities of Napaeozapus found in this study were in an area of New York State having boreal elements. The lower densities reported for western and southern areas of the northeastern United States indicate that its overall distribution is affected by the presence of boreal vegetation. Ad libitum water consumption shows that redback voles drink more than twice as much as their predicted weight—relative value. Woodland jumping mice drank normal weight—relative amounts, but deer mice drank less. The rate of evaporation in Napaeozapus was considerably lower than that found in sympatric Peromyscus maniculatus. Its rate of evaporation was closer to values found for some populations of mice from drier climates. These results suggest that moisture is not a critical limiting factor in the distribution of woodland jumping mice. The low rate of evaporation in Napaeozapus could be an adaptation against desiccation during hibernation. The spreading of saliva by the deer mice is an important cooling device. At 37°C deer mice lost 72% of their heat production through evaporation. At this same ambient temperature, jumping mice lost only 38% of their heat by evaporation. Deer mice were able to withstand 39°C with no ill effects, but few jumping mice survived 37°C. Hyperthermia associated with low metabolism was observed in the deer mice. This could be the result of vasoconstriction in nonvital organs, thereby limiting substrates and O2to the cells of these organs. Such an adjustment would not only limit the rate of metabolism but would also increase the body temperature by limiting transfer of heat by the blood. The decreased metabolic rate and the increased rate of evaporation by spreading saliva would increase the efficiency of cooling at high ambient temperatures. Basal metabolic values of deer mice and jumping mice are near their predicted weight—relative values. The high lower critical temperatures in jumping mice are consistent with the idea that hibernators have high rates of heat loss. Jumping mice appear to have more precise thermoregulation over a wider range of ambient temperatures than do deer mice during the summer period. Metabolic patterns of several small rodents are compared. These patterns show little association with specific climatic conditions among distantly related, sympatric forms. The deer mouse complex, the jumping mice, the voles, and the pocket mice seem to have their own distinctive metabolic and thermoregulatory patterns, which may be associated with phylogenetic patterns, as exemplified by populations of Peromyscus, while some of the more specialized phylogenetic patterns, such as the one which characterizes the genus Perognathus, seem to have evolved in the ancestral populations to suit specific climatic conditions.