Apomorphine and the putative dopamine agonist, 2-(N, N-dipropyl)-amino-5, 6-dihydroxytetralin induced dose-dependent climbing behaviour in the mouse which was measured in wire mesh lined cages as the percentage of time spent climbing in the 30 min period following the first climb and as the maximum time spent in a single climb throughout the drug effect. These These two measures were generally found to parallel excepting when the interacting agent caused muscular hypotonia. All potential interacting agents were given as pretreatments to determine changes in motor function which may interfere with the climbing induced by 1.0 mg/kg s.c. apomorphine. The possibility of a change in the apomorphine response to a sterotyped biting, which would also interfere with climbing, was also considered. Excluding these non-specific changes, climbing behaviour was shown to be antagonised, dose-dependently, by low doses of typical and atypical neuroleptic agents (haloperidol, fluphenazine, loxapine, pimozide, oxiperomide, clozapin, thioridazine, sulpiride, tiapride and metoclopramide) but not specifically by other psychoactive agents. Climbing behaviour was modified by serotonergic agents; the agonist quipazine reduced or abolished, whilst the antagonists, methysergide and cyproheptadine, enhanced the response. Picrotoxin specifically reduced climbing behaviour but sodium valproate exerted non-specific effects, precluding conclusions as to a GABA involvement. Cholinergic and noradrenergic involvements with climbing were also apparently eliminated by the ineffectiveness of atropine, aceperone, piperoxan and propranolol. The involvement of serotonin with climbing was extended to the actions of the neuroleptics: the antagonistic effects of typical neuroleptics (haloperidol, fluphenazine, loxapine) were markedly enhanced by combination with methysergide or cyproheptadine whilst the effects of clozapine, sulpiride and thioridazine were significantly reduced. The actions of metoclopramide, oxiperomide, pimozide and tiapride were not generally modified by such combinations. These differences are discussed in terms of differential abilities to induce extrapyramidal disturbances and the mouse climbing model is forwarded as a test with potential to detect antipsychotic agents of different activity spectra.