Using a retrograde tracer technique with protein horseradish peroxidase (HRP), attempts were made to determine afferent projections to the nucleus raphe dorsalis (NRD). As a control, the injection of the HRP was also made into one of the following structures adjacent to the NRD: (1) mesencephalic periaque ductal gray; (2) nucleus linearis intermedius; and (3) third cranial nucleus. The present results indicate that the NRD, particularly is rostral part, receives direct projections arising from: (1) locus coeruleus complex (locus coeruleus, locus coerulus alpha, and locus subcoeruleus); (2) parabrachial nuclei (nucleus parabrachialis lateralis and medialis); (3) nucleus laterodorsalis tegmenti; (4) griseum centrale pontis, particularly the caudal part of the nucleus incertus; (5) substantia nigra; (6) lateral habenular nucleus; (7) hypothalamic areas, particularly dorsal and lateral hypothalamic areas; (8) preoptic areas; (9) anarea dorso-lateral to the inferior olivary complex and medial to the lateral reticular nucleus; and (10) raphe nuclei; particularly nucleus linearis intermedius, centralis superior, pontis and magnus. The present findings thus confirm some previous reports on the afferent projections to the NRD described in the cat and rat, and further indicate the richness of afferent connections of the NRD. Some problems of the peroxidase technique have also been discussed.