Abstract: The distribution of the substance P-like immunoreactive (SP-LI) neurons in the basal forebrain cholinergic corticopetal neuron areas in the cat was studied and compared with that of the choline acetyltransferase-like immunoreactive (ChAT-LI) neurons in the paired consecutive sections of the corresponding areas.A continuum of ChAT-LI neurons were distributed in the medial septal nucleus (MS),nuclei of diagonal band,vertical and horizontal divisions (NDBV/H),nucleus basalis of Meynert (NBM),subcommissural ventral pallidum (sVP),and the areas of interstitial elements of Ch4,e.g.the internal capsule (ic) and nucleus of anterior commissure (Nac).The distribution of the SP-LI neurons closely resembled that of the ChAT-LI neurons with similar shape and size in the corresponding areas of the MS-NDB-NBM continuum.However,no perikarya in the paired surfaces of two adjacent sections could be really identified as bisected neurons being SP- and ChAT-LI respectively.The number of the SP-LI neurons,compared with that of the ChAT-LI neurons was more in MS,almost the same in NDBV/H,much less in NBM,and only a few in the interstitial elements of Ch4 areas and sVP.The results demonstrated that the majority of the SP-LI neurons in the MSNDB-NBM were mainly contained in the MS-NDB and most of the SP-LI neurons in the cholinergic neuron areas belonged to a subdivision with different chemical property.We suggested that the Spergic neurons in the MS-NDB might be one of the main components,in addition to the cholinergic and GABAergic neurons,in the septohippocampal projection system and might play an important role in modulation of the memory-functioning neocortex-hippocampusneocortex circuit.