Adult male wild rabbits were exposed to artificial daylengths of 8L:16D, 13L:11D or 20L:4D after being taken from the wild during September, when testicular size was minimal. In all groups the testes grew and those in 20L:4D grew more quickly than did those in 8L:16D or 13L:11D. Rabbits kept in the 3 daylength treatments for the whole study maintained enlarged testes after the initial growth phase. The testes of rabbits transferred to 13L:11D after 8 weeks in 20L:4D did not regress. A similar transfer after 16 weeks in 20L:4D resulted in testicular regression (P less than 0.01) followed by moult, and then testicular regrowth 9 weeks after the change of daylength. No regression occurred at this time in the control group kept in 20L:4D throughout the study. Plasma FSH concentration increased at the beginning of the study and was elevated during the initial period of testicular growth. Values then declined after the asymptote of testicular weight was reached. Plasma testosterone and LH concentrations did not change significantly throughout the study. The results showed that reproductive regression in wild rabbits can be induced by a reduction in the daylength, but that the potency of this reduction is dependent upon previous experience of daylength.