Effects of the dietary carbohydrate-to-fat ratio on opiate-stimulated eating and on naloxone-, cholecystokinin- and bombesin-suppressed eating were examined. Rats were fed either a high carbohydrate (cornstarch) diet (68% of energy from carbohydrate and 12% fat), an intermediate diet (40% carbohydrate and 40% fat) or a high fat (corn oil and lard) diet (3% carbohydrate and 77% fat). Other rats self-selected from the high carbohydrate and high fat diets. Subcutaneous administration of naloxone, an opiate antagonist, generally suppressed intake of the high fat diet to a greater extent than intake of the high carbohydrate diet. Neither cholecystokinin octapeptide nor bombesin (administered intraperitoneally) exerted preferential suppression of fat intake. The opiate agonists ketocyclazocine and butorphanol tartrate administered subcutaneously at 1000 h preferentially, although not exclusively, stimulated intake of the high fat diet in a dose-dependent manner during the 6-h feeding trial. Repeated daily subcutaneous injections of butorphanol tartrate caused rats to consume more than 50% of their daily intake during the 6-h period postinjection; intake during the normal night feeding period was suppressed to maintain total daily intake equal to that of vehicle-injected rats. We conclude that stimulation of the opioid feeding system contributes to the overeating often associated with consumption of a high fat diet.