The testicular weight response of 2 wk old male voles exposed to 10, 12, 14, 15 or 16 h of light (L)/day for 4 wk and of 5 wk old males exposed from birth to 12, 13, 13.5, 14, 14.25, 14.5, 14.75 or 15 L/day was proportional to the photoperiod; 13L or less produced little growth, but 13.5L or more produced progressively heavier testes. Only 14.5L and 15L gave full spermatogenesis. Night-break schedules produced a marked testicular response with 7L:8D h of dark:1L:8D and 7L:6D:1L:10D, intermediate effects with 7L:4D:1L:12D and none with 7L:2D:1L:14D. Of various skeleton photoperidos with 0.75L only 0.75L:12.5D:0.75L:10D and 0.75L:10.5D:0.75L:12D produced marked increases in testicular weight. The peak of photosensitivity appeared to be phase-shifted towards dawn by reducing the initial light phase. Its position appeared to move from 16 h after dawn in block photoperiods to 14 h after dawn in the night-break and 12 h after dawn in the skeleton photoperiod experiments. Previous experience of 16L:8D or 8L:16D had little effect on the subsequent response to skeleton photoperiods (0.5L:9D:0.5L:14D, 0.5L:13D:0.5L:10D, 0.5L:15D:0.5 L:8D). Animals reared in these photoperiods only showed a positive testicular response to 0.5L:13D:0.5L:10D. The lack of response to constant darkness indicated that voles were capable of perceiving these skeleton photoperiods.