Fourteen rats were trained to discriminate the effects of gamma-hydroxybutyrate (GHB) (sodium salt, 200 mg/kg) and saline in a two-lever choice task using a fixed ratio 10 schedule of water reinforcement. Intermediate responding, i.e., responding not fully appropriate for either training condition was observed in tests following morphine, lysergic acid diethylamide, chlordiazepoxide, and the presumed GABA-mimetics muscimol, gamma-butyrolactone, baclofen, and 3-aminopropane sulfonic acid. Naloxone blocked the intermediate results following morphine, but had no effect on GBH-induced stimulus control. The GABA antagonist bicuculline partially blocked GHB, but pizotyline, phentolamine, and butaclamol were without effect. It is concluded that the compound stimulus produced by GHB is most closely associated with GABAergic systems, but that minor opiate and serotonergic components are present as well.