Multiple responses to different photoperiods occur in the mouse, Peromyscus leucopus.

1. Peromyscus leucopus exhibits multiple responses to different daylengths. The winter molt and gonadal regression are complete in populations chronically exposed to 12, 11, 10, or 9 h of light per day while hypertrophy of brown fat and occurrence of daily torpor were greater in mice on 9 or 10 h of light when compared to animals on 11 or 12 h photocycles. 2. A precise "critical" daylength is not observed in this species for gonadal regression, occurrence of daily torpor, or hypertrophy of brown fat. 3. Certain mice never exhibit a short-day response following 12 weeks exposure to 12, 11, 10, or 9 h of light per day.