Rats given long-term access to a palatable sucrose solution are more sensitive to (1) the analgesic potency of morphine and (2) the anorectic actions of naltrexone. In the present experiment, the ability of naltrexone to antagonize the antinociceptive properties of morphine was examined in sucrose- and chow-fed rats. Twenty adult male Long-Evans rats were maintained on chow and water. Ten rats were also given ad libitum access to a 32% sucrose solution for three weeks. Baseline and post-drug nociceptive thresholds were determined with a warm water (52 degrees C) tail-withdrawal procedure. Complete morphine dose-effect curves (0.32-100.0 mg/kg) were determined alone and in the presence of naltrexone (0.32-0.32 mg/kg). Morphine produced significant increases in tail-withdrawal latencies in both groups of animals. Rats drinking sucrose had significantly longer tail-withdrawal latencies than rats given only chow and water. Naltrexone produced significant and surmountable dose-dependent rightward shifts in the morphine dose-response curves for both groups, but did not differ in potency across diets. The present data support and extend our earlier findings showing sucrose enhancement of morphine-induced antinociception in rats given a palatable sucrose solution to drink.