By employing both in vivo and in vitro labeling techniques, opiate receptors were labeled with tritiated diprenorphine in the monkey brain and localized by light microscopic autoradiography. Both methods of labeling gave similar results, allowing a description of discrete areas having opiate binding sites. High concentrations of opiate receptors were found in the substantia gelatinosa of the spinal cord, nucleus tractus solitarius, area postrema, lateral parabrachial nucleus, substantia grisea centralis, several nuclei of the thalamus and hypothalamus, substantia innominata and in the amygdala. In the monkey pituitary, receptors were found in the neurohypophysis. These results correlate well with those found in autoradiographic studies of the rat brain although there are a few notable differences. Many of the opiate receptor distributions can be correlated with anatomical loci of brain functions known to be influenced by administration of opiate compounds.