In order to analyze the presence of a reelin-like protein in the brain of a primitive vertebrate with a laminar-type brain, such as the sea lamprey, Western blot and immunohistochemical approaches were employed by using the G10 and 142 reelin-specific monoclonal antibodies. Western blots of lamprey brain extracts showed bands of about 400 kDa, 180 kDa and others below 100 kDa; similar bands were observed in samples from rat cerebellum. In different larval stages there was a prominent reelin immunolabeling associated with the olfactory bulb, pallial regions, habenula, hypothalamus and optic tectum. In addition, the olfactory and optic tracts, as well as the afferent and efferent (fasciculus retroflexus) tracts of the habenular ganglion, also showed immunopositivity in these stages. Interestingly, the highest level of labeling was observed in premetamorphic larvae, just prior to entering the metamorphic stage. These data indicate that reelin expression is also prominent in brains of primitive vertebrates without layered cortical regions, suggesting that some physiological roles of reelin not related to the regulation of neuronal migration in layered cortical regions (i.e. involvement in axon pathfinding, synaptogenesis, dendritic arborization and neuronal plasticity) might have appeared earlier in evolution. © 2002 Elsevier Science B.V. All rights reserved.