The p35/cdk5 neuronal-specific kinase complex has been shown to play an important role in the laminar configuration of cortical neurons. Mice lacking either p35 or cdk5 exhibit a disrupted cortical lamination pattern. We showed previously that instead of the normal "inside-out" layering pattern of cortical neurons, cortical neurons are layered from "outside-in" in p35 mutant mice. To gain insight into the mechanisms that underlie these defects, we examined the organization of landmark structures formed during cortical development and the migratory behavior of p35(-/-) cortical neurons by using bromodeoxyuridine labeling. In the present study, we show that reelin localization in the marginal zone is normal in p35 mutant mice. Furthermore, the preplate splits into the marginal zone and subplate properly, a developmental event that fails to occur in reeler mice. Finally, the migration of the earliest born cortical plate neurons is normal in p35 mutant mice; cortical neurons subsequently generated remain underneath these neurons. These data suggest that the p35/cdk5 kinase is required for cortical plate neurons to migrate past preexisting neurons and take up superficial positions to constitute the inside-outside layering order of cortical lamination.