The retrograde transport-HRP-immunocytochemical technique was employed to ascertain if the periaqueductal gray-raphe magnus projection arises from neurons containing somatostatin, neurotensin, serotonin or cholecystokinin. Following HRP injections into the raphe magnus (NRM) double-labeled cells containing HRP reaction product and somatostatin-, neurotensin- or serotonin-like immunoreactivity were identified in the midbrain periaqueductal gray (PAG). No cholecystokinin-like immunoreactive double-labeled neurons were found in the PAG. These results indicate that the PAG-NRM pathway contains somatostatin, neurotensin and serotonin but not cholecystokinin.