Target size, i.e. body size appropriate for age, may be reset by bilateral lesions of several brain areas. The mechanism for control of target body size is unknown, but some of the loci have marked effects on gustatory behavior and/or energy metabolism. We have tested the possibility that a disturbance in energy metabolism may be a common factor in resetting target size. Food efficiency for body weight gain and for metabolic size (the 0.75 power of body weight) was determined in rats that were experimentally stunted by neonatal head-irradiation or by bilateral electrolytic lesions produced soon after weaning in the dorsomedial hypothalamic nuclei (DMH) or the substantia nigra (SN). The irradiations were carried out in males and females; the surgical lesions were produced only in males. Observations were carried out from weaning through early adulthood. Subgroups of irradiated rats and controls were fasted for 48 hours at 40 days of age. Irradiated rats had reduced food efficiency for weight gain and for metabolic size, more marked in males than in females. DMH or SN lesions did not change food efficiency for weight gain. Food efficiency for metabolic size increased after DMH lesions and declined after SN lesions. During refeeding after a fast, irradiated rats showed a normal transient increase in food efficiency for weight gain, but not for metabolic size. The differences in food efficiency following different lesions tend to exclude altered energy metabolism as a common factor in the reset of target body size.(ABSTRACT TRUNCATED AT 250 WORDS)