Rats were trained to discriminate injections of racemic pentobarbital (5.0 mg/kg) from saline in a two-lever drug discrimination task. After stable discrimination performance was attained, stimulus generalization studies were conducted with another barbiturate (barbital), benzodiazepine derivatives (diazepam and chlordiazepoxide), pyrazolopyridine derivatives (etazolate, cartazolate, and tracazolate), and a triazolopyridazine (CL 218, 872). The pentobarbital stimulus generalized to all of these compounds, except cartazolate. In addition, the administration of the benzodiazepine receptor antagonist flumazepil prior to benzodiazepine or triazolopyridazine administration produced a dose-related antagonism of each generalization. In contrast, the administration of flumazepil before barbiturate or pyrazolopyridine (i.e., etazolate or tracazolate) injection resulted in no attenuation of these generalizations. The results indicate that while certain barbiturates, benzodiazepines, pyrazolopyridines and triazolopyridazines are capable of producing similar stimulus effects, the behavioral actions of these agents can be differentiated on the basis of their susceptibility to antagonism by flumazepil.