These studies document species differences in the distribution of the peptide substance P and the catecholamine-synthesizing enzyme tyrosine hydroxylase (TH) within a central nervous system region of a number of mammalian species including the mouse, rat, guinea pig, rabbit, cat, and two species of hamster (Chinese and Syrian). Substance P-containing neuronal perikarya were observed in the main olfactory bulb (MOB) of both species of the hamster, but not in the MOB of the other species examined. In the accessory olfactory bulb (AOB), however, neuronal staining was observed in all species except the mouse. The number of stained somata and their intensity varied such that label was most prominent in the rat followed in decreasing order by the rabbit, guinea pig, cat, and hamster. The mouse displayed no perikaryal staining. Stained somata in AOB were found in the internal granule cell layer with dendritic processes ramifying through the internal plexiform layer to arborize within the mitral cell layer. The distribution of substance P-stained neurons in the MOB also differed between the two hamster strains. In the Syrian hamster, neurons were primarily juxtaglomerular. In the Chinese hamster, labeled perikarya were found in both the juxtaglomerular region and within the superficial aspect of the external plexiform layer (EPL). The mean longest diameter of the majority of substance P-labeled neurons in both species was greater than 10 micron, suggesting that they were tufted cells. Those in the EPL of the Chinese hamster were the largest (17 micron). Species differences also were observed in the distribution of substance P-positive axons and terminals within the MOB. Label was distributed primarily in the internal granule cell layer of the Syrian hamster and the internal plexiform layer of the Chinese hamster. Tyrosine hydroxylase staining was similar among species with the exception of the Syrian hamster. In the latter species, an additional large population of neurons was found within the external plexiform layer. In all other species, TH-stained neurons were found scattered throughout the MOB and occasionally the AOB but were not numerous in the EPL. Although most TH neurons were larger than 10 microns, in all species a population of smaller TH cells was observed primarily in the glomerular layer, suggesting that most neurons labeled with TH are tufted cells but that some may be periglomerular cells.(ABSTRACT TRUNCATED AT 400 WORDS)