The 24 h resting (postabsorptive) energy expenditure of adult female lean and obese Zucker rats was examined both at the body weights each genotype spontaneously maintained and at body weights that were reduced 6-10 percent by restricting food intake for 2 weeks to 70 or 55 percent of normal. At their normally-maintained body weight, lean rats expended energy at a daily rate appropriate to their body mass according to the Kleiber equation (predicted kcal/day = 70.0 x BW-0.75kg observed kcal/day = 70.66 x BW-0.75kg). However, expenditure rates declined significantly in the weight-reduced lean rats restricted to 70 or 55 percent of normal intake (to 64.1 and 61.3 kcal/kg0.75). The obese rats expended energy at a rate lower than predicted by the Kleiber equation prior to weight loss (49.5 x BW-0.75kg). But, rates of daily expenditure in the obese rats was further reduced by 70 or 55 percent restriction (to 43.7 and 42.4 x BW-0.75kg), a decline proportionately as large or larger than that displayed by restricted lean rats. With this reduced metabolic rate, the total daily caloric expenditure of obese rats restricted to 55 percent of normal intake approximated that of unrestricted lean rats weighing only half as much (27.8 vs 27.5 kcal/day; 569 vs 285 g). Evidently, Zucker obese rats display the same adaptive reductions in resting metabolism as do leans in response to energy deficit and weight loss. In both, these adjustments in expenditure act to stabilize body weight at the levels that typify each genotype.