Sexual development is inhibited in Siberian hamsters (Phodopus sungorus) in short days (SD), and a small uterus is an obvious indicator of photo-inhibition. The small uterus in SD is presumably due to the delayed onset of estrous cycles. However, in an earlier study, the investigators reported that serum estradiol (E2) concentration was significantly higher in young females raised in SD than in long days (LD), with the highest concentrations measured in SD at 4 wk of age. These seemingly contradictory findings were investigated in the present study. First, uterine mass and body mass were measured in SD-and LD-reared hamsters from 1 to 12 wk of age. Uterine mass was significantly greater in LD than in SD by 3 wk of age and onward. Thereafter, our investigation focused on 4-wkold hamsters. Serum E2 concentrations in LD and in SD were not significantly different and there were no significant LD-SD differences in uterine estrogen receptors (ER), as measured by immunohistochemistry and quantitative real-time RT-PCR. Therefore, alternative explanations for the photoperiodic difference in uterine size in young Siberian hamsters are considered.