Noradrenaline (NA), dopamine (DA) and DOPAC were determined with a newly developed radioenzymatic method simultaneously in the striatum, limbic system, hypothalamus and in catecholamine-containing cell groups of the rat brain. Only a loose relationship could be established between DOPAC and DA contents in the various brain areas. The lowest relative DOPAC level (DOPAC/DA ratio) was found in the median emience, while it was the highest in the periventricular nucleus of the hypothalamus. Haloperidol increased the DOPAC level in only part of the nuclei examined (striatum, olfactory tubercle, central amygdaloid nucleus), while in other limbic regions as well as in the hypothalamic dorsomedial, arcuate and paraventricular nuclei it proved to be ineffective. The DOPAC level in the locus coeruleus was decreased by haloperidol. Pargyline caused an appr. 50% decrease of DOPAC content of most of the nuclei in 10 min; the effectivity of the drug did not show parallelism with that of haloperidol. The monoamine oxidase inhibition caused no change in the DOPAC level in the hypothalamic periventricular and paraventricular nuclei. Results are discussed as a consequence of different reactivity of various DA-ergic terminals and catecholamine cell bodies to haloperidol and pargyline.