A combined method of retrograde tracing of horseradish peroxidase (HRP) and serotonin (5-HT) immunocytochemistry indicated that some 5-HT-like immunoreactive neurons in the midbrain periaqueductal gray (PAG) of the rat send their axons to the nucleus accumbens. These PAG neurons are mainly located in the ventromedial and ventrolateral subdivisions at the middle and caudal levels of PAG.