Some behavioral and biochemical effects of four dopaminergic agonists (apomorphine, piribedil, lergotrile, and bromocriptine) were determined in the mouse. As expected, all four drugs dose-dependently reversed the alpha-methyltyrosine-induced decline of forebrain dopamine. All four compounds reduced locomotor activity at low doses, but only apomorphine and bromocriptine increased motor activity at higher doses. All four drugs caused some reversal of the baclofen-induced elevation in forebrain dopamine concentrations, but only apomorphine and bromocriptine completely reversed the effects of baclofen. After chronic treatment with haloperidol, the behavioral effects of lergotrile and bromocriptine were altered. Doses of those drugs reducing motor activity in normal animals were ineffective after chronic haloperidol. The latent stimulation induced by bromocriptine was enhanced, while a stimulatory effect of lergotrile emerged in these animals. These effects were noted in conjunction with an enhanced sensitivity to the drug-induced decrease in dopamine turnover. These results demonstrate that dopamine agonists may be differentiated on the basis of certain behavioral and biochemical tests and suggest an interaction of these drugs with two different populations of dopamine receptors.