The distribution of opiate receptors, determined by the autoradiographic localization of stereospecific [3H]diprenorphine binding sites, was studied in the brain stem. Areas showing very dense or dense localizations of receptors included the parabrachial nuclei, the superior colliculus, the ventral median raphe nucleus, components of the accessory optic system, portions of the habenulo-interpeduncular complex, the pretectal nuclei and the ventral lateral geniculate, the infundibulum and the medial thalamus. Moderate grain densities were found in broad areas of the upper medulla, midbrain and diencephalon. The significance of these findings are discussed in terms of pain mechanisms, association of some receptors with small diameter axons, pupillary reflexes, hormonal control, and effects of opiates on neurotransmitter systems.