The beta-carboline FG 7142 was studied alone and in combination with Ro 15-1788, CGS 8216 and lorazepam in squirrel monkeys trained to respond under a fixed-interval (FI) schedule of food presentation. FG 7142 (0.3-5.6 mg/kg i.v.) produced dose-related decreases in the rate of FI responding, effects opposite to those of moderate doses of lorazepam (0.03-0.3 mg/kg i.v.). Pretreatment with low doses of Ro 15-1788 (0.1 and 0.3 mg/kg i.v.) shifted the dose-response curve of FG 7142 progressively to the right indicating pharmacological antagonism at benzodiazepine recognition sites. In comparison, pretreatment with the pyrazoloquinoline CGS 8216 (0.1-1.0 mg/kg i.v.), which alone decreased responding, did not alter the effects of FG 7142 in a systematic manner. Combinations of behaviorally active doses of FG 7142 and lorazepam had primarily additive effects: the opposing actions of one drug tended to cancel the other's effect on responding. These results show that the reduction in behavior by FG 7142 is modified predictably by Ro 15-1788 but not by CGS 8216, and behaviorally active doses of both FG 7142 and lorazepam may be needed for their mutual antagonism.