Previous studies have shown a possible connection between the nucleus raphe dorsalis (NRD) and the amygdala in mediating opioid analgesia. In the present study, horseradish peroxidase (HRP) retrograde tracing was used in combination with serotonin (5-HT) immunocytochemical staining in an attempt to search for serotonergic projections from the NRD to the amygdala. In rats which received an injection of HRP into the amygdala, HRP retrogradely labelled 5-HT-immunoreactive cells were observed in the NRD. About 10% of the 5-HT-immunoreactive neurons in the NRD give rise to axons to the amygdala. These cells are predominantly situated in the ipsilateral wing and ventromedial part of the NRD. These data indicate the existence of serotonergic projections from the NRD to the amygdala, providing a morphological substrate for the putative antinociceptive pathway from the NRD to the amygdala.