In this study immunohistochemistry is used to investigate the distribution of opioid peptides, substance P (SP), neuropeptide Y (NPY) and acetylcholine (ChAT) in the olfactory tubercle (OT) of the cat. On the basis of our histochemical findings we divide the OT into two parts: the cortical part and the cap regions, the latter containing the granule cell islands. The cortical part shows an intensely ir-punctate staining pattern (opioid and SP), similar to that observed in the striatum, to which it is connected via plexus bridges. The pyramidal neurons representing the main cell type of the cortical part are ir-negative. NPY-ir neurons invade the OT via the plexus bridges from the striatum and are restricted to the cortical part. A different staining pattern exists in the cap regions: dwarf and small pyramidal-like cells display opioid- and SP-like immunoreactivity and therefore are clearly separated from the cortical part. The intensely stained axonal plexus of the cap-region neurons occupies the hilus regions dorsal to the granule islands. In addition, dendrites of large pallidal neurons densely enmeshed in opioid- and SP-ir fibers (woolly fibers) enter the OT from the dorsally located ventral pallidum, pass through the hilus, traverse the granule islands and reach the dwarf cell layer, where the ir-axons apparently terminate. The granule islands do not receive ir-terminals and the granule cells are ir-negative, except some SP-positive granules in the medial islands. Within the hilus regions some large neurons are ChAT-positive, but the majority is ir-negative. The hilus neurons are regarded as the main target of the cap region efferent system. The findings of this study parallel and confirm our morphological observations (Meyer and Wahle 1986).