Angiotensin converting enzyme is localized in the monkey (Macaca fascicularis) brain by in vitro autoradiography using the radiolabelled inhibitor, [125I]351A. This radioligand binds with high affinity and specificity to monkey cortical sections. Specific inhibitors of converting enzyme, lisinopril and perindoprilat compete for the radioligand binding to monkey cortex sections with inhibitory constants of 10 nM. High concentrations of angiotensin converting enzyme occur in most components of the basal ganglia including the caudate nucleus, putamen, the internal and external globus pallidus, nucleus accumbens, ventral pallidum and the reticular part of the substantia nigra. The distribution of converting enzyme in the caudate nucleus and putamen is heterogeneous, with prominent patches of higher activity. The patches in the caudate nucleus correspond closely with the acetylcholinesterase-poor striosomes. In the hypothalamus, very high levels of angiotensin converting enzyme occur in the median eminence and the pituitary stalk and high concentrations occur in the supraoptic and suprachiasmatic nuclei. Moderate, diffuse binding is observed in the median preoptic nucleus, the medial preoptic area, and in the anterior, lateral, ventromedial, posterior and arcuate nuclei. In the circumventricular organs, the subcommissural and subfornical organs exhibit high levels of angiotensin converting enzyme. The organum vasculosum of the lamina terminalis and the pineal body display moderate enzyme activities whereas the area postrema is devoid of labelling. The interpeduncular nucleus and, in the hippocampal formation, the molecular layer of the dentate gyrus are also intensely labelled. High levels of angiotensin converting enzyme activity are also detected throughout the cerebral cortex with laminations of higher activity corresponding to cell dense layers of the cortex. Layered binding is also present in the cerebellar cortex, with the most intense labelling in the molecular layer. Moderate concentrations of converting enzyme also occur in the paraventricular, medial habenula, lateral habenula and central median nuclei of the thalamus, the amygdala, the central gray, the locus coeruleus, the parabrachial nucleus and dorsal tegmental nucleus. The dorsal vagal complex, inferior olivary nucleus and the caudal subnucleus of the spinal trigeminal nucleus all display high levels of binding. Moderate, diffuse labelling is found throughout the reticular region and is also present in the gracile and cuneate nuclei. Although the overall distribution of angiotensin converting enzyme in the monkey brain resembles that in the rat, there are some striking differences. These include the high levels of binding throughout the monkey cerebral cortex and in the interpeduncular and suprachiasmatic nuclei.