The possibility that factors in addition to motivational or psychomotor deficits contribute to body weight losses following lateral hypothalamic (LH) lesions was assessed in 2 experiments. Rats with LH lesions failed to gain weight when compared to sham-operated controls, when equal quantities of nutrients were given by intragastric feeding. They also lost weight more rapidly than controls under total starvation conditions regardless of whether food was present or absent in the G1 tract prior to surgery. These results could not be explained on the basis of differences in urine or fecal output or activity levels. An increase in core temperature was found in LH lesioned rats immediately following surgery and throughout the experimental period. These results suggest that lateral hypothalamic lesions induce a metabolic impairment.