Photoperiodic regulation of testicular function was investigated in homozygous tau mutant hamsters; these animals have an innate circadian period of about 20 h. In 20-h light:dark (LD) cycles, the minimum photoperiod required to prevent testicular regression was between 10.0 and 11.5 h per 20-h cycle (equivalent to 12.0-13.8 circadian hours). This was proportionally similar to the minimum photoperiod necessary to prevent regression in wild-type hamsters maintained in 24-h LD cycles. To examine the shape of the photoperiodic photosensitivity curve in homozygous tau mutant hamsters, the authors measured the effects of different T cycles on testicular maintenance. Entrainment to LD 1:18.0 and LD 1:20.5 partially or completely prevented gonadal regression in homozygous tau mutant hamsters, but LD 1:19.4 did not prevent regression. When considered in terms of circadian time, the photoperiodic photosensitivity curve for homozygous tau mutant hamsters was similar to that described previously for wild-type hamsters. The results indicate that, as in wild-type hamsters, photoperiodic regulation of reproduction is regulated by circadian photosensitivity in homozygous tau mutant hamsters. Because tau mutant hamsters measure day length against a time base of 20 h, the circadian pacemaker that measures day length might be the same as that which generates circadian rhythmicity in locomotor activity. The authors' data leave open the question of whether the tau mutation has had effects on the control of reproduction that are not directly attributable to its effects on the period of the circadian oscillator.