The technique of in situ hybridization with specific ribonucleotide probes was used to determine the distribution patterns of mRNA encoding the alpha 1a-, alpha 1b- and alpha 1d-adrenoceptor (AR) subtypes in rat brain and spinal cord. The expression pattern of alpha 1a-AR mRNA has not been reported previously, and was found to be widespread throughout the rat central nervous system. High levels were found in regions of the olfactory system, several hypothalamic nuclei, and regions of the brainstem and spinal cord, particularly in areas related to motor function. Regions expressing moderate levels of mRNA for this receptor were the septum, bed nucleus of the stria terminalis, cerebral cortex, amygdala, cerebellum and pineal gland. Low expression levels were detected in the hippocampal formation. Most nuclei in the basal ganglia and thalamus expressed extremely low or undetectable levels of alpha 1a-AR mRNA. The expression patterns of the alpha 1b- and alpha 1d-AR mRNAs were similar to those described using oligonucleotide probes in earlier studies. High expression of alpha 1b-AR mRNA was noted in the pineal gland, most thalamic nuclei, lateral nucleus of the amygdala and dorsal and median raphe nuclei. Moderate expression levels were noted throughout the cerebral cortex, and in some olfactory, septal, and brainstem regions. The distribution of alpha 1d-AR mRNA was the most discrete of the three receptors examined. Expression was strong in the olfactory bulb, cerebral cortex, hippocampus, reticular thalamic nucleus, regions of the amygdala, motor nuclei of the brainstem, inferior olivary complex and spinal cord. Comparison of the distributions of the alpha 1a-, alpha 1b- and alpha 1d-AR mRNA suggests unique functional roles for each of these receptors.