The reproductive cycle of male red-sided garter snakes (Thamnophis sirtalis parietalis) was studied in a laboratory-maintained population and in nature. Both populations exhibited a dissociated reproductive pattern in which spermatogenesis was initiated after the breeding season and sperm were stored in the vas deferens over winter. Although animals in both the laboratory and the field exhibited a dissociated reproductive pattern, temporal differences were evident in the two populations. Animals maintained in the laboratory initiated and completed spermiation much sooner than animals collected in the field. In the captive population, the renal sexual segment (RSS) could be identified but did not exhibit a seasonal fluctuation, indicating perhaps that androgen production was continuous, but at a reduced level. In nature, the RSS was hypertrophied at emergence, regressed following the breeding season, and again hypertrophied as circulating androgens increased in the fall, prior to hibernation. In both laboratory-maintained and field animals the circulating concentrations of testosterone and dihydrotestosterone at emergence were initially similar to the levels observed prior to hibernation but by 2 weeks postemergence had decreased to a relatively low level. Both the high androgen levels prior to hibernation and the rapid decrease during the spring courtship season may result from temperature influences on clearance rates. Plasma levels of corticosterone varied during the spring, first increasing upon emergence and then declining approximately 2 weeks following emergence.