Testis size in male wild rabbits (Oryctolagus cuniculus) kept in an enclosure in Cambridgeshire, U.K. was maximal during May and June and minimal between October and December. Regression occurred after the summer solstice and recrudescence occurred after the winter solstice. Rabbits kept in long days for 25 wk showed no sign of spontaneous testis regression. Hence, testis regression during the summer is probably not due to development of refractoriness to long days. Testis regression occurred in rabbits transferred from long (16L:8D) to short (8L:16D) days. Within 8 wk of the transfer spontaneous regrowth of the testes occurred and the rabbits moulted, and after 16 wk the testes had recovered to their size before the transfer. Subcutaneous implants of melatonin given to rabbits in long days mimicked the effect of a transfer to short days by causing testis regression followed by recrudescence. Moult occurred in rabbits immediately after short day- or melatonin-induced testis regression. The study demonstrates that seasonal reproduction in male wild rabbits in Britain is largely controlled by changes in photoperiod and that this is probably mediated via the pineal gland.