We studied the effect of the opioid receptor antagonist naloxone on intake of three isocaloric diets containing cornstarch, sucrose, or Polycose as the predominant carbohydrate in ad libitum-fed and food-restricted rats. A large body of evidence suggests that opioids affect palatability (reward)-rater than hunger (energy deficit)-driven food intake. We expected food intake to be driven by both energy needs and palatability in ad libitum-fed rats, whereas in food-restricted rats we expected intake to be driven by energy needs with a relatively small palatability component in the preferred sucrose and Polycose diet groups. In the ad libitum-fed rats, naloxone significantly reduced nocturnal intake of all three diets at doses of 0.3, 1.0, and 3.0 mg/kg. In contrast, naloxone failed to alter intake of the cornstarch diet in chronically food-restricted rats. However, naloxone decreased intake of the sucrose diet in food-restricted rats at doses of 0.3, 1.0, and 3.0 mg/kg and decreased intake of the Polycose diet at the 3 mg/kg dose. These data lend further support to the notion that opioids are involved in some other component of feeding than that induced by energy needs.