In lizards and snakes, the kidneys produce seminal fluid in addition to having osmoregulatory functions. Therefore, in response to polyandry, kidney mass should be under selection regimes like those influencing testes. Male red-sided garter snakes deposit a kidney-derived copulatory plug that functions in sperm competition. We first tested for intersexual differences in kidney mass and allometry in one population and found that males had kidneys twice as heavy as those of females, consistent with stronger selection on male kidney mass. Previous studies have shown that male size enhances mating success in one-on-one competition prevalent in small mating aggregations. We then examined the relationship between body size, kidney mass, and testes mass in two populations with low (LD) and high (HD) mating aggregation densities. Males from the HD population had heavier testes and heavier kidneys than did males from the LD population; HD males were also smaller in body size. Our results suggest that the strength of sexual selection, and possibly the balance between pre- and postcopulatory selection, varies in response to population size or density. However, more replication is needed to confirm the generality of these results within red-sided garter snakes and other squamate reptiles.