To examine the potential extrinsic sources of substance P (SP)-containing terminals in the C1 area of the RVL, immunoperoxidase localization of SP was combined with retrograde transport of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP). In all cases examined, several neurons containing SP and WGA-HRP were found in the nucleus raphe pallidus and a few dual labeled neurons were found in the nucleus of the solitary tract and the paraventricular nucleus of the hypothalamus. In some cases, one or two double labeled neurons were detected in the lateral hypothalamic area and nucleus raphe obscurus. The results demonstrate that although several brain structures contribute SP afferents to the RVL, the nucleus raphe pallidus is the major extrinsic source.