We examined the effect of a single live-trap capture on body mass of deer mice (Peromyscus maniculatus) by comparing body mass of individuals observed in artificial burrows (no capture stress) with body mass recorded <24 h later following a single capture in a live trap. Nearly 90% of our 374 observations showed changes in body mass, and 90% of these 336 changes were losses in body mass. Changes in body mass were affected by age, gender, season of capture, and reproductive condition of adult females. Abiotic factors also affected changes in body mass of mice in traps. Subsequent observations of individuals in burrows suggested that body mass changes due to trap confinement were short-term impacts, as differences found in body mass between consecutive observations of adult deer mice in artificial burrows did not differ significantly from zero. However, numerical differences in mass between burrow observations were directly related to initial mass lost in live traps.