Reproductive behavior of the eastern garter snake (Thamnophis sirtalis sirtalis) was evaluated in the field at the Mosquito Hill Nature Center, New London, Wisconsin. In addition, twenty-eight males (24 adult; 4 subadult) were collected during the active season to examine the reproductive physiology. Courtship behavior and mating in this population was similar to previous reports on Thamnophis with the exception of a relatively abbreviated courtship season lasting only 3–5 d. Our study population exhibited a dissociated reproductive pattern in which spermatogenisis was initiated after the breeding season had ended and sperm over wintered in the vas deferens. Upon emergence from low temperature dormancy, the testis mass and seminiferous tubules were regressed with spermatogonia the dominant cell type. Maximum tubule and epithelial development occured in late summer corresponding to peak spermatogenesis and elevated testosterone levels (T). While the tubule diameter and epithelial height of the renal sexual segment (RSS) did not exhibit seasonal fluctuation, sexual granules within the cells of the epithelia did display a seasonal variation that was determined by the absolute number, individual granule density and staining properties. Testosterone levels (T), elevated in the fall before entering winter dormancy, were not significantly different from the levels of animals sampled immediately upon emergence. There was, however, a significant reduction in circulating T in animals sampled following the end of the breeding season. Four small males collected in mid-to-late summer had just initiated spermatogenesis. Examination of the caudal vertebrae by skeletochronology revealed these individuals were in their second active season. In these subadults, development of the testes and RSS development was not as advanced as that of adult males collected at the same time. In addition, the level of circulating T in the subadults was significantly lower than in adults collected at the same time.