Rats maintained under restricted access to food (but at 100% free-feeding weights) received one of two diets in their home cages: a palatable sucrose-based diet, or regular chow (grain based diet), and could respond for either sucrose- or grain-based reinforcers under an FR 40 reinforcement schedule (trossover design). Naloxone (0, 0.1, 0.3, 1.0, and 3.0 mg/kg) was more potent in reducing operant-chamber responding in rats maintained on a sucrose-based diet in their home cages than those fed a grain-based diet, regardless of the type of pellets available in the operant chambers. Whereas naloxone decreased response rate over the session, it had no effect on initiation of responding. Results support the hypothesis that opioids are involved in the maintenance, but not the initiation of consummatory behavior. Furthermore, increased potency of naloxone following chronic ingestion of palatable food is similar to that observed following chronic opiate administration, suggesting a relationship between palatability and opioids.