We collected plasma samples from adult males in a natural population of Plains Gartersnakes (Thamnophis radix) to characterize seasonal variation in testosterone. As in other New World temperate zone natricines, testosterone was high during spring emergence, declined as courtship and mating progressed, and then increased in late summer and fall. Based on patterns seen in this and other studies, we suggest that the rapidity with which testosterone decreases following spring emergence may be associated with the length of the mating season; and that it decreases rapidly in species and populations in which the mating season is brief and more slowly in species and populations in which mating occurs over an extended period. Testosterone showed a clear, positive association with body condition in adult male Plains Gartersnakes. Whether this association translates into greater reproductive success warrants further investigation. Because we collected blood samples multiple times from individual males within years, we were also able to demonstrate significant individual variation in seasonal testosterone profiles, although how this might relate to individual variation in reproductive behavior is unknown. Given their phylogenetic, ecological, and geographic diversity, New World natricine snakes represent a rich resource for studying hormonal associations with reproductive patterns.