Forty derivatives (1-40) of pyrazolo[1,5-a]pyrimidine were synthesized and evaluated for antianxiety properties via gross behavioral observations in rats. Five of these compounds, including 5,7-dimethylpyrazolo[1,5-a]pyrimidine (6) and the 3-fluoro (7), 3-chloro (8), 3-bromo (9), and 3-iodo (10) derivatives, were selected for advanced evaluation. Although 6 and 7 had marginal activity, 8-10 had an anxiolytic effect in animals comparable to the clinically useful benzodiazepines, diazepam, and chlorodiazepoxide. Comparison with chlorpromazine indicated that 6-10 are probably not antipsychotic agents. These compounds also lacked activity in anticonvulsant and analgesic tests. Acute toxicity data (mouse, ip and po) indicated that 8-10 had excellent therapeutic ratios, although 10 was more poorly absorbed than 8 and 9. Further demonstration of anxiolytic efficacy was obtained by comparing the effects of 8 and 9 with the benzodiazepines in modifying provoked aggression in monkeys, rats (muricide), and fighting mice. The most remarkable observation, however, was that 8 and 9 had no effect, at the anxiolytic threshold, in potentiating the CNS depressant effects of ethanol or sodium barbital (po) in treated mice. In contrast, diazepam and chlorodiazepoxide potentiated this drug interaction effect at minimal anxiolytic doses.