Negative effects due to infection have not been documented for most natural host-parasite associations. I performed an infection experiment to measure the physiological consequences of infection by Capillaria hepatica in deer mice (Peromyscus maniculatus gracilis). Compared to their uninfected siblings, infected mice displayed increased serum concentrations of alkaline phosphatase and total proteins and reduced concentrations of albumin. Infected mice also had higher hematocrits and enlarged livers and spleens. There was no difference in swimming endurance between infected and control animals, possibly because mice did not perform at maximum aerobic capacity. Sex differences in alkaline phosphatase and albumin levels suggest that females are more sensitive to similar burdens of C. hepatica. Differenct hematocrit responses between mice from 2 localities suggest that historical association can lead to evolutionary change in pathophysiology caused by this nematode. Further experiments would reveal whether these disturbances lead to reductions in host fitness.