Abstract: Removal of a single testis results in hypertrophy of the remaining gonad (compensatory testicular hypertrophy; CTH) in juveniles and adults of many mammalian species. CTH is thought to be mediated in part by post-castration decreases in negative feedback from the testes onto the hypothalamus and/or pituitary and the resulting increased FSH secretion. Although this phenomenon has been used to investigate hypothalamic-pituitary-gonadal (HPG) signaling mechanisms, seasonal modulation of CTH has been largely ignored. Because sensitivity to testicular feedback inhibition of gonadotropins varies seasonally in hamsters, I tested the hypothesis that day length affects the CTH response. CTH was evident in long-day (LD) housed Syrian and Siberian hamsters unilaterally castrated in adulthood or as juveniles (Ch. 3, 4, and 5). In contrast, juvenile and adult hamsters maintained in short day lengths (SDs) did not exhibit CTH while displaying the SD phenotype of regressed testes (Ch. 3, 4, and 6). SD non-responsive and photorefractory hamsters did develop CTH (Ch. 6). CTH was accompanied by hypersecretion of FSH in LD juvenile and SD refractory Siberian hamsters. In contrast, FSH hypersecretion was absent in SD photosensitive juvenile Siberian hamsters that did not exhibit CTH (Ch. 3). Thus, photoperiod modulates the testicular response to unilateral castration; SDs prevent the CTH response. This is consistent with the notion that sensitivity to testicular negative feedback in SDs is increased to such a degree that one testis produces sufficient hormonal feedback in short but not long days to restrain FSH secretion without requiring increases in testicular cell size or number. Previous studies established species differences with respect to CTH under various conditions. Specifically, laboratory rats and mice do not display CTH after post-pubertal hemicastration, whereas the majority of mammals tested, including hamsters, exhibit robust CTH regardless of age at surgery. Also, whereas the CTH response is absent in Syrian and Siberian hamsters with regressed testes (Ch. 3, 4, and 6), rams display CTH in non-breeding photoperiods. Thus, different species may utilize different physiological mechanisms to regulate testicular function. This emphasizes the importance of comparative research and illustrates the dangers associated with conclusions drawn from research conducted on one or two species.