The study of intra- and inter-individual variation in the metabolic response to environmental variation can provide mechanistic explanations to large-scale ecological and evolutionary patterns. In a study of range-limiting factors, variation in resting metabolic rates of cottonmouths (Agkistrodon piscivorus leucostoma) was investigated along a latitudinal gradient in southern populations and in populations near and at the northern range limit. CO(2) production rates of 53 snakes were measured in response to body mass, temperature, time of day, latitude of origin, and sex. The within-subjects effects were similar to those reported for other pit vipers. Metabolic cold adaptation appears to exist, with cottonmouths from northern populations having higher low temperature metabolic rates. Calculations suggest that Arkansas cottonmouths allocate almost twice as much energy to resting metabolism during non-feeding periods (brumation) as Louisiana cottonmouths. While maintenance metabolism alone during brumation is more costly near the northern range limit, it is most likely not a limiting factor in geographic distribution and may be used to fuel important processes other than activity metabolism.