The distribution of substance P-like (SP) and vasoactive intestinal polypeptide-like (VIP) structures was studied using an indirect immunoperoxidase technique in the mediterranean barbel Barbus meridionalis and in the rainbow trout Salmo gairdneri. SPlike positive fibers were observed in the inner strata of the mediterranean barbel olfactory bulb, mainly the granule cell and the plexiform layers. Ganglion cells and fibers of the terminal nerve were also labeled. No SP-positive structure was found in the ol- factory bulb of the rainbow trout. On the contrary, the VIP antiserum used displayed very strong immunostaining in the olfactory nerve fiber layer of Salmo gairdneri, whereas those fibers in Barbus meridionalis showed no immunoreactivity. After a complete transection of the olfactory tract and a survival time of 20 days, SP-immunostained fibers were not observed. Thus, they can presumably be identified as centrifugal fibers, coursing from the telencephalic hemispheres through the olfactory tracts, into the olfactory bulb. The VIP immunoreactivity was confined to the olfactory fibers, both in the olfactory nerve and the olfactory bulb. The positive immunostaining disappeared after chemical lesion of the ol- factory mucosa. These observations demonstrate that the olfactory bulb of freshwater teleosts exhibits a high degree of heterogeneity in its immunocytochemical distribution pattern, this pattern also differing from previous reports on higher vertebrates.