A survey of the literature indicates that previous workers (Osgood, 1909; Nicholson, 1941; Hamilton, 1943; Cahalane, 1947; Burt, 1949) have been aware of the change from the gray juvenile pelage to the “rufous” or cinnamon red of the adult deer mouse, Peromyscus leucopus noveboracensis , but no investigation has been made of the progress of the pelage change or the factors that control it. Collins (1918), studied pelage change in Peromyscus maniculatus gambeli and several other closely related western forms. The present study was initiated with a two-fold purpose; (1) to follow the course of the pelage change; (2) to correlate age with pelage change. The data presented here are from observations made on deer mice captured in the Ithaca, New York, region during the summers of 1949 and 1950 and from animals of the same subspecies taken in the Cincinnati, Ohio, area during the spring and summer of 1954. All observations were made on litters born and raised in captivity. All litters were from pregnant females captured in the field. No attempt was made to breed the mice in captivity. In order that environmental conditions might be as natural as possible, the animals were kept in large outdoor cages placed directly on the ground. Each cage, constructed of ¼inch hardware wire, was three feet long and one foot square in cross section, and equipped with a large one-foot-square wooden nesting box. Tall grass and weeds soon grew up through the wire mesh and provided excellent cover for the animals. Since these mice are extremely nervous, they were observed and handled as infrequently as possible. Before the onset of pelage change the animals were never handled, although daily observations of the nest were made. When checking the pelage, the mice were caught and examined on …