Food intake over 90 min post-injection was studied in groups of food deprived (20 hr) female Sprague-Dawley (S.D.) rats, fatty Zucker (fa/fa) rats and their heterozygous lean litter mates (Fa/fa) of various ages, that bar-pressed for food pellets on a FR-1 schedule following a subcutaneous (SC) or an intracerebroventricular (IVT) injection of sterile saline or naloxone HCl. Subcutaneous injections of naloxone HCl (10 mg/kg) reduced feeding in all three groups of rats compared to SC saline; in addition, a greater percentage reduction in food intake over the whole 90 min test period occurred in the fa/fa rats given SC naloxone compared to the Fa/fa group. Intracerebroventricular naloxone (50 micrograms) decreased feeding over the initial 30 min period in the S.D. and Fa/fa rats but a 100 micrograms IVT dose was needed to reduce feeding in the fa/fa group. The results demonstrate that central naloxone administration can suppress feeding in both non-obese and obese strains of rats as it is known to do when given peripherally. These findings add yet further evidence to the premise that endogenous opioid peptides may play an intricate and important physiological role in the regulation of feeding behavior.