We evaluated effects of age, sex, season, and year on body mass of endangered San Joaquin kit foxes ( Vulpes macrotis mutica ) over a 16-year period at the Naval Petroleum Reserves in California. Mean body mass of kit foxes increased by 35–47% for the 1st 3 months of life, and foxes gained ca. 90% of their total adult mass by 10 months of age; no significant changes in mass occurred after 22 months of age. Males were significantly heavier (5.8%) than females by 3 months of age, and by 10 months of age and afterwards, males were ca. 20% heavier than females. Adult foxes (≥22 months) lost ca. 3.5% of their mass between winter and the following summer, but reasons for this seasonal change in mass are unknown. Annual variation in mass of kit foxes was detected and seemed related to variation in availability of prey.