Sections at regular intervals through the human thalamus and alternating with those used for histochemical and cytoarchitectonic analysis in the companion paper, were stained immunocytochemically with a monoclonal antibody that recognizes tachykinins, including substance P, and with an antiserum against metenkephalin-arg-gly-leu (MERGL). Immunoreactivity for both types of molecule is found in fiber systems that enter the thalamus: (a) anteromedially, from the hypothalamus and prethalamic regions; (b) posterolaterally, from the midbrain tegmentum. No immunoreactive somata were observed in the thalamus. The chief nuclei in which dense, apparently terminal, ramifications of both sets of immunoreactive fibers are found include the posterior complex (Po, L and Sg), the intralaminar nuclei except the centre median, and a small nucleus tentatively identified as the nucleus submedius. It is significant that many of these nuclei have been implicated in pain phenomena, that most of them receive spinothalamic inputs, and that spinal cells at the origins of the tract are immunoreactive for substance P or metenkephalin. This study provides further evidence for the close similarity between nuclear delineation in the human and monkey thalami.