The telencephalon in ray-finned fish (actinopterygians) is everted, in contrast to the evaginated telencephalic hemispheres in all other vertebrates. In the more derived ray-finned fish, the teleosts, proliferation of neurons and their migration from the ependymal zone of the pallium renders comparisons between telencephalic cell groups of the teleosts and members of other vertebrate groups extremely difficult. The telencephalon of Polypterus (a primitive living ray-finned fish), although everted, is cytoarchitecturally much simpler than that of teleosts. We have thus applied immunohistochemical techniques to the study of the telencephalon of Polypterus to help clarify the evolution of the telencephalon in teleosts and facilitate comparisons between the telencephalon in ray-finned fish and other vertebrates. Antisera against the following neuroactive substances were used: 1) serotonin (5HT), 2) tyrosine hydroxylase (TH), 3) substance P (SP), 4) leucine-enkephalin (ENK), 5) neuropeptide Y (NPY), and 6) the neurotensin-related hexapeptide LANT6. Several features of the labeling patterns obtained suggested that the dorsal and ventral subdivisions of the area ventralis are homologous as a field to the basal ganglia and septum plus other basal telencephalic regions of land vertebrates, sharks and lungfish: 1) an abundance of SP+, NPY+, and ENK+ fibers; 2) an abundance of TH+ fibers, possibly of posterior tubercle/tegmental origin; 3) the presence of an SP+ fiber bundle that appeared to descend from basal telencephalic levels and terminate in the posterior tubercle/tegmentum, which contain TH+ (possibly dopaminergic) neurons; and 4) an abundance of 5HT+ fibers, presumably of posterior tubercle/tegmental origin. It was not possible, however, to recognize distinct pallidal and striatal subdivisions within the area ventralis of Polypterus. The olfactory pallium (P1) was generally poor in most of the substances examined, except for the presence of LANT6+ fibers. The P3 pallial field was conspicuously rich in SP+ and ENK+ fibers throughout its extent, and the caudal and lateral parts of the P2 field were rich in SP+ fibers and ENK+ fibers. Since this is characteristic of the medial pallial and/or dorsomedial pallial walls of the telencephalon in lungfish, sharks, frogs, and reptiles, the P3 field and caudolateral part of the P2 field may be homologous to these portions of the telencephalon in other vertebrates. More rostromedial parts of P2 may correspond to those parts of the pallium in land vertebrates that are in receipt of specific sensory input from the thalamus, since low neuropeptide levels are characteristic of these regions.(ABSTRACT TRUNCATED AT 400 WORDS)