Angiotensin II and angiotensin III in the brain exert their various effects by acting on two pharmacologically well-defined receptors, the type-1 (AT1) and the type-2 (AT2) receptors. Receptor binding autoradiography has revealed the dominant presence of AT1 in brain nuclei involved in cardiovascular, body fluid and neuroendocrine control. The cloning of the AT1 complementary DNA has revealed the existence of two receptor subtypes in rodents, AT1A and AT1B. Using specific riboprobes for in situ hybridization, we have previously shown that the AT1A messenger RNA is predominantly expressed in the rat forebrain; in contrast the AT1B subtype predominates in the anterior pituitary. Using a similar technical approach, the aim of the present study was to establish the precise anatomical localization of cells synthetising the AT1A receptor in the adult rat brain. High AT1A messenger RNA expression was found in the vascular organ of the lamina terminalis, the median preoptic nucleus, the subfornical organ, the hypothalamic periventricular nucleus, the parvocellular parts of the paraventricular nucleus, the nucleus of the solitary tract and the area postrema, in agreement with previous autoradiographic studies, describing a high density of AT1 binding sites in these nuclei. In addition, AT1A messenger RNA expression was detected in several brain areas, where no AT1 binding was reported previously. Thus, we identify strong expression of AT1A messenger RNA expression in scattered cells of the lateral parts of the preoptic region, the lateral hypothalamus and several brainstem nuclei. In none of these structures was the AT1B messenger RNA detectable at the microscopic level. In conclusion, it is suggested that angiotensins may exert their central effects on body fluid and cardiovascular homeostasis mainly via the AT1A receptor subtype.