Ex vivo D-1 or D-2 receptor binding in the striatum was reduced by 65-78% after treatment with EEDQ (N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline) in combination with either the D-2 antagonist, raclopride, or the D-1 antagonist, SCH 23390, respectively. EEDQ induced a 65% reduction in D-1 receptor binding and a 51% decrease in cAMP production in striatal homogenates. Selective D-2 receptor inactivation inhibited the stereotyped behaviour induced by the mixed D-1/D-2 agonist, apomorphine, or by the D-2 agonist, quinpirole, when given alone and in combination with the D-1 agonist, SK&F 38393. Selective inactivation of D-1 receptors did not inhibit the behavioural effects of quinpirole when given alone and in combination with the D-1 agonists, SK&F 81297, SK&F 38393 or SK&F 75670. Likewise, the effect of apomorphine was unchanged. These results indicate that a normal density of D-2 receptors is critical for the expression of the stereotyped behaviour induced by DA agonists. In contrast, there is a large surplus of D-1 receptors to enable the response to a D-2 agonist. This is particularly illustrated by the persistent behavioural effects of the partial D-1 agonist, SK&F 75670, in rats with up to a 78% decrease in D-1 receptor binding.