Rats were trained to respond under 3-min fixed-interval schedules of food presentation, and effects of the benzodiazepine-receptor ligands, flumazenil, 2-(4-methoxy-phenyl)-pyrazolo[4,3-c]quinolin-3(5H)-one (CGS 9895), 3-carbo-t-butoxy-beta-carboline (beta-CCtB), and beta-carboline-3-carboxylic acid ethyl ester (beta-CCE) were assessed before and after the induction of tolerance to chlordiazepoxide. Before daily administration of chlordiazepoxide, none of the antagonists produced appreciable effects on rates of responding up to doses of 32.0 mg/kg i.p. beta-CCE was the only antagonist studied at a higher dose (100.0 mg/kg i.p.), which decreased response rates. After 23 days of daily chlordiazepoxide administration (oral doses started at 10 and increased to 100 mg/kg/day by the 17th day), dose-effect curves for chlordiazepoxide were shifted to the right by about one-half log unit. Subjects were also more sensitive to the flumazenil, CGS 9895, and beta-CCtB, however, since these drugs produced only small effects in non-tolerant subjects, precise estimates of the degree of the shift in dose-effect curves could not be estimated. However, there were differences in the changes in the dose-effect curves induced by chlordiazepoxide tolerance. These results suggest differences in mechanism of action of antagonists in tolerant and non-tolerant subjects, and further that the sensitivity that is induced to antagonists in tolerant subjects is not conferred equally to all drugs having benzodiazepine antagonist activity.