Contaminant exposure is an environmental factor often overlooked in ecological studies. Yet exposure to toxicants is likely to be metabolically expensive, and result in a trade-off between energy spent to detoxify an excrete contaminants, and energy allocated to growth or reproduction. To quantify the energetic cost of PCB exposure, we fed mice diets containing 0, 0.1, 10 and 25 ppm PCBs (mg PCBs/kg food). After 6 wks, there were no differences in food intake or body mass related to the level of dietary PCBs. We continued to feed mice the PCB diets for one year, and then measured food intake and resting metabolic rate. After one year, all mice had gained mass, but mice on the 25 ppm diet were significantly heavier than mice in the other groups. Mice on the 25 ppm diet also showed an increase in both food intake (p=0.06) and metabolic rate (p=0.01), after correction for differences in body mass. Food intake averaged 4.1 g/d for mice on the 25 ppm diet, compared to 3.7 gld for control mice. Oxygen consumption at 30°C averaged 40.1 ml O2/h for mice on the 25 ppm diet, compared to 36.6 ml O2/h for control mice. These results show that for white-footed mice, there is an energetic cost to contaminant exposure that when combined with other environmental stresses, will influence patterns of energy acquisition and allocation.