Total caloric intake and patterns of dietary self-selection of the three macronutrients, protein, carbohydrate and fat, were examined in adult male rats maintained on a 6-hr feeding schedule following daily injections of morphine (10 mg/kg), naloxone (1 mg/kg), the two drugs together, and saline. Animals received drug injections for 10 consecutive days. All animals received saline injections for the 5 days preceding and 5 days following the experimental period. Naloxone injections led to a significant reduction in total caloric intake. Neither morphine nor morphine and naloxone together significantly affected total caloric intake. Each of the drugs had a distinct effect on macronutrient selection. Morphine produced a significant increase in fat intake and decrease in carbohydrate intake, while naloxone led to a slight reduction in fat intake. When the two drugs were given together, a significant elevation in carbohydrate intake and reduction in fat intake were observed. Protein intake was not affected by any of the drugs. These results are discussed with respect to the hypothesized role of the endogenous opioid system in the regulation of energy balance.