The annual cycle of testes weight of adult male rabbits (Oryctolagus cuniculus) was studied in three populations on the sub-Antarctic Kerguelen archipelago during 1984–85. This study was completed in April 1987 with an analysis of differences in diet quality between populations, assessed by the proportion of nitrogen and lignin in stomach contents. A marked annual cycle of testes weight was observed in all populations, with a rapid fall in testes weight from February to March. As in most rabbit populations studied elsewhere, testes growth resumed immediately in two populations (called Morne and Molloy). In contrast, testes growth was deferred by five to six months in the third population (Armor), subject to harsher environmental conditions. Moreover, a larger proportion of the Armor males had inactive testes during the reproductive season. The diet of the Armor population was characterized by low nitrogen and high lignin content, suggesting poor-quality diet as an immediate cause of deferred testes growth. We propose that reduced testes size during adverse conditions might be adaptive by increasing survival probability through reduction of basal metabolism and limitation of agonistic and reproductive activities, behaviours which are costly in terms of high energetic expenditure. However, at the population level, the timing of the breeding season was not related to the cycle of male testes weight and thus seems to depend on females' condition.