Bilateral intra-accumbens 6-OHDA (2 micrograms in the presence of DMI and tranylcypromine, 14th postoperative day) enhanced the climbing responses of mice to apomorphine and 2-(N,N-dipropyl)amino-5,6-dihydroxytetralin causing parallel shifts of the normal log dose-response curves to the left. The enhancement of the apomorphine response was shown to be dependent on the dose of 6-OHDA, 0.5 micrograms being threshold and 2 micrograms maximum. Increased climbing was apparent by the 5th postoperative day, maximum by the 10th day, and was then maintained throughout the experimental period (6-8 weeks). 0.25-2 micrograms intra-accumbens 6-OHDA caused dose-related decreases in the dopamine content of mesolimbic areas (nucleus accumbens and tuberculum olfactorium) without causing significant changes in mesolimbic noradrenaline or striatal dopamine. In the absence of DMI/tranylcypromine, 2 and 4 micrograms 6-OHDA also decreased mesolimbic noradrenaline and striatal dopamine content. 16 micrograms 6-OHDA injected into the striatum (after DMI/tranylcypromine) decreased the striatal dopamine content by 85% (without altering mesolimbic dopamine or noradrenaline content) but this treatment failed to modify apomorphine climbing (2nd-12th postoperative days). Haloperiod, sulpiride, thioridazine, clozapine ad metoclopramide each caused a dose-dependent decrease in apomorphine climbing in both normal and 6-OHDA-treated mice. Haloperidol and metoclopramide were approximately equipotent in both groups of animals whilst sulpiride and thioridazine were approximately 4x more potent in the 6-OHDA-treated mice (the development of muscular hypotonia made an interpretation of clozapine effects difficult). The data indicate that an important role for the mesolimbic nucleus accumbens in the mediation of apomorphine climbing, and indicate that the antagonism by sulpiride and thioridazine may be specifically increased when mesolimbic mechanisms are rendered 'supersensitive'.