Short day lengths or reduced food availability are salient cues for small mammals that breed seasonally. Photoperiod-mediated gonadal regression in white-footed mice (Peromyscus leucopus) is a slow, orderly process that involves testicular apoptosis. Testicular regression in response to restricted caloric intake is relatively rapid, and it is generally reversed quickly by ad libitum (ad lib) feeding. To determine the contribution of apoptotic cell death during food restriction, and to examine possible interactions with photoperiod, mice housed in long (16L:8D) or short (8L:16D) photoperiods were fed either ad lib or 70% of their average ad lib intake. Testes were removed at 2, 4, 6, or 8 wk of experimental treatment. Apoptotic activity was determined by in situ TUNEL labeling and assessment of DNA laddering. Significant (P < 0.05) gonadal regression in response to short days was first detected at 8 weeks in mice fed ad lib. Food-restricted, long-day mice also showed significant testicular regression at 8 wk. Combined exposure to short day lengths and food restriction resulted in significant testicular regression at 6 wk (P < 0.05). TUNEL labeling was slightly, though significantly, elevated in germ cells at 2 and 4 wk in long-day food-restricted mice (P < 0.05). TUNEL labeling was also elevated in short-day food-restricted males at these early times but then increased nearly 5-fold at 6 and 8 wk in these mice (P < 0.001). DNA laddering confirmed elevated apoptosis. Overall apoptotic activity negatively correlated with paired testis mass, plasma testosterone, and spermatogenic index measurements in both ad lib and food-restricted males. Few histological markers of necrotic cell death were observed in any group. Taken together, these results suggest that testicular regression in response to limited caloric intake or short days is mediated by apoptosis.