Using in situ hybridization, expression of the mRNA for reelin, the gene most probably responsible for the reeler trait, was studied during mouse brain development. from embryonic day 13 to maturity. The highest level of expression was found in Cajal-Retzius neurons, while a high signal was also seen in the olfactory bulb, the external granular layer of the cerebellum and, particularly at early developmental stages, in hypothalamic differentiation fields, tectum and spinal cord. A moderate to low level of expression was found in the septa1 area, striatal fields, habenular nuclei, some thalamic nuclei, particularly the lateral geniculate, the retina and some nuclei of the reticular formation in the central field of the medulla. Paradoxically, no reelin expression was detected in radial glial cells, the cortical plate, Purkinje cells, inferior olivary neurons and many other areas that are characteristically abnormal in reeler mutant mice. Together with other preliminary studies, the present observations suggest that the action of reelin is indirect, possibly mediated by the extracellular matrix. Most of the data can be explained by supposing that reelin is a cell-repulsive molecule which prevents migrating neurons from invading reelin-rich areas, and thus facilitates the deployment of radial glial cell processes and the formation of early architectonic patterns.