The telencephalon of teleost fish shows high affinity uptake for D-[3H]aspartate, intermediate levels of GABAergic markers and low levels of cholinergic enzymes. Experimental results (resection of the olfactory tracts or unilateral kainic acid administration in the telencephalon) suggest that: the projection from the olfactory bulbs to telencephalic targets is mediated by aspartate and/or glutamate; and a population of GABAergic neurons is present in a telencephalic area which is considered homologous to part of the striatal complex of land vertebrates. From the present results, it appears that the neurochemical approach can be used, in conjunction with neuroanatomical methods, to study evolutionary problems of telencephalic function.