Discriminative control was established among morphine, saline and naitrexone in rhesus monkeys receiving morphine every other day. Three hours prior to sessions subjects received saline or 3.2 mg/kg morphine; immediately prior to sessions they received saline or 0.01 mg/kg of naitrexone. There were dose-related generalizations to each training condition: morphine generalized to the morphine plus saline lever; small doses of naitrexone reversed effects of morphine and larger doses occasioned responding on the morphine plus naitrexone lever; in one monkey still larger doses occasioned responding on the saline plus saline lever. When saline was administered 3h earlier, naitrexone had no effect in one subject and occasioned responding on the morphine plus naitrexone lever in a second subject. Nalbuphine substituted for morphine plus saline in one monkey and for morphine plus naitrexone in a second monkey; ketamine did not substitute for either training drug. That stimulus control was established between no drug and a combination of morphine and naitrexone suggests the latter condition did not represent the absence of morphine. In addition to demonstrating stimulus control for three conditions in rhesus monkeys, the current study suggests opioid antagonists might have novel discriminative stimulus effects at opioid receptors even under conditions where signs of withdrawal are not evident.