The distribution of neurons in the rat CNS that synthesize mRNA for the α(1A/D) and α(1B) adrenoceptors was revealed by the in situ hybridization method. Forty-eight-mer DNA probes were synthesized to two different and unique regions of both the α(1A/D) and α(1B) mRNAs. Tissue sections from all levels of the CNS and some peripheral ganglia were incubated in a hybridization cocktail containing one of these four probes. The two mRNAs were expressed in a discrete and often complementary manner to each other, and identical hybridization patterns were seen for the probes directed against the same mRNA. The α(1A/D) probes hybridized heavily with neurons in the internal granular and internal plexiform layers of the olfactory bulb, in layers II-V of most areas of the cerebral cortex, and in the lateral aspect of the lateral amygdaloid nucleus, with pyramidal neurons of CA1-CA4 regions, hilar and granular neurons of the dentate gyrus, and neurons in the reticular thalamic nucleus, cranial and spinal motor nuclei, and the inferior olivary nucleus. Light labeling was seen in a variety of other regions in the brain and spinal cord. The α(1B) probes hybridized heavily with neurons in the mid layers of cerebral cortex and with virtually all neurons in the thalamus, except the reticular and habenular nuclei. In addition, labeling was seen in the lateral and central amygdaloid nuclei, in brainstem and spinal motor nuclei, over most neurons of the dorsal and medullary raphe nuclei and neurons of the intermediolateral cell column in the spinal cord. Light labeling was seen in the septal nucleus, the horizontal limb of the diagonal band, the paraventricular and lateral hypothalamic nuclei, the pontine and medullary reticular formation, and in most laminae in the spinal cord. The patterns of labeling obtained with the α(1B) probes resemble the labeling seen in previous autoradiographic ligand binding studies utilizing 'general' α1 ligands, while the labeling patterns seen with the α(1A/D) probes do not correspond to any published α1 receptor distribution pattern, indicating that this mRNA likely encodes for a novel adrenoceptor. The present findings further expand the heterogeneity of adrenoceptor mRNAs presented in two accompanying studies (Nicholas et al., 1993a,b). This differential distribution of adrenoceptors subtypes provides a framework for the functional diversity to the apparently widespread, diffuse, and rather homogeneous noradrenergic innervation of the CNS.