Exercise stimulates reproductive function in hamsters exposed to short-day photoperiod (SDP) in contrast to its inhibitory effects in women and rats. SDP inhibits hamster reproduction in part by increasing the sensitivity of the hypothalamo-pituitary-gonadal axis (HPGA) to the negative feedback of gonadal steroids. To determine whether EX facilitates reproduction in female hamsters by affecting this mechanism, we examined the influence of estradiol (E2) on basal LH and FSH concentrations in exercising and sedentary hamsters maintained on long-day photoperiod (LD 14:10, LDP) or SDP (LD 8:16). In the LDP, serum LH and FSH were unaffected or reduced by exercise in ovariectomized (OVX) nonhormone-replaced hamsters, and LH was increased after tonic E2 replacement compared to sedentary controls. In the SDP, serum LH and FSH were significantly higher in OVX exercising than in sedentary hamsters, whether the exercisers were injected with a high dose of E2 or not. Thus, the effects of exercise on basal gonadotropin secretion in female hamsters appear to depend on the level of estradiol negative feedback (ENF). When this feedback is low (LDP OVX condition), exercise is either ineffective or inhibitory. When the ENF is increased by exposure to SDP and/or by treatment with E2, exercise has a stimulatory effect on basal gonadotropin secretion. Exercise may stimulate hamster gonadotropin secretion by reducing the ENF either by lowering the sensitivity of the HPGA to steroid negative feedback or by other means.