Changes in body mass of a pair of captive Japanese Rock Ptarmigan Lagopus mutus japonicus and their chicks were measured using an automatic weighing system at Omachi Alpine Museum from 13 January 1995 to 29 October 1996. The male's body mass decreased gradually in late April, the beginning of the breeding season, but returned to the normal level in the fall. In contrast, the female's body mass increased rapidly from late April onward, peaking in late May, just before the egg-laying period. Thereafter, the female's body mass decreased throughout the egg-laying, incubation, and early brooding periods, but began to increase again in the fall. In 1996, four chicks were successfully reared and their body mass increased linearly during the first 60 days after hatching, followed by a milder increase.