Recent clinical evidence suggests that the tricyclic antidepressant imipramine is effective against not only panic disorder but also generalized anxiety disorder. Although most animal models of anxiety appear to be insensitive to this agent, such work has almost invariably employed an acute treatment regimen. In the present study, ethological methods have been used to assess in detail the effects of acute and chronic imipramine treatment on the behaviour of male DBA/2 mice in the elevated plus-maze test. In contrast to acutely administered diazepam (1 mg/kg), which produced a significant anxiolytic profile on standard and ethological measures, neither acute nor chronic (daily, 15 days) treatment with imipramine (0-20 mg/kg) was associated with anxiety reduction. Data are discussed in relation to test sensitivity factors and the nonspecific mechanism of action of imipramine.