The storage and utilization of energy plays a critical role in reproductive output for females of many species, including snakes. However, links between energy and reproductive effort in males are less clear. Studies at a communal den of red-sided garter snakes (Thamnophis sirtalis parietalis (Say, 1823)) in Manitoba suggest that energy is critical to reproduction for males as well as females. Males vary substantially in body condition (mass relative to body length) at the time they emerge from winter inactivity. The energy to be expended in courtship is stored in the muscles rather than the "conventional" sites for energy storage in snakes (abdominal fat bodies or liver). A male's reproductive effort (the duration of his residency at courting aggregations near the den) was linked to his energy stores and to the rate of depletion of those stores. Male snakes that emerged from hibernation in better condition, and that lost mass slowly thereafter, remained in courting aggregations near the den for longer periods than did males that emerged in poor condition (i.e., with less energy resources) and (or) lost mass more rapidly. In outdoor arenas, males that engaged in courtship lost mass more rapidly than did males with no courtship opportunities. These data suggest that courtship is energetically expensive for male garter snakes and that the amount of effort that a male invests in reproduction is determined by his energy stores.