Behavioral effects of the putative endogenous benzodiazepine receptor ligand, inosine, were studied alone and in combination with the benzodiazepine antagonist Ro 15-1788. Keypeck responses were maintained by food under a multiple fixed-interval 3-min, fixed-interval 3-min schedule of food delivery. Under the multiple schedule, the first response after 3 min produced food in the presence of either white (no punishment) or red keylights and, in addition, each 30th response produced a brief electric shock (punishment) when the keylight was red. Inosine increased the low rates of punished responding (10-100 mg/kg IM) and the higher rates of unpunished responding (30 mg/kg). The benzodiazepine antagonist Ro 15-1788 (0.03 mg/kg, IM) antagonized the rate-increasing effects of inosine but had no effect when given alone. Combinations of inosine (30 mg/kg) with higher doses of Ro 15-1788 (0.1-1 mg/kg) decreased responding much like Ro 15-1788 alone. The marked rate-decreasing effects of 1000 mg/kg inosine were not affected by concurrent administration of Ro 15-1788 (0.01-1 mg/kg). The behavioral effects of inosine alone resembled effects of benzodiazepines but not those of benzodiazepine antagonists. The response rate-increasing effects of inosine may be due to its benzodiazepine receptor binding properties, whereas the rate decreases produced by higher doses of inosine appear to be unrelated to benzodiazepine receptors.