Unilateral injections of agents that are useful tools for differentiating electrophysiologically distinct dopamine receptors within the snail Helix aspersa are given into behaviourally distinct dopamine sensitive areas within the caudate nucleus of cats. Dopamine-elicited contralateral head-turning is selectively mimicked by apomorphine and selectively inhibited by haloperidol, whereas dopamine-elicited homolateral head-turning including oro-facial dyskinesias is selectively mimicked by (3, 4-dihydroxy-phenylamino)-2-imidazoline (DPI) and selectively inhibited by ergometrine, piribedil and noradrenaline. These results indicate that the caudate nucleus of cats contains two functionally and pharmalogically distinct dopamine receptors and that Helix aspersa dopamine sensitive neurons can be used as model for the design of drugs selectively interfering with each type of receptor. The implications of these findings are considered in view of the efficacy of piribedil, L-DOPA and neuroleptics in psychomotor diseases in man.