Cellular compartmentation was studied in the germinal matrices and the intermediate zone of the cerebral cortex of rat embryos that survived for 1 or more days after injection with [3H]thymidine. In contrast to the vertical compartmentation seen in the neuroepithelium with short-survival thymidine autoradiography, sequential-survival autoradiography revealed a horizontal compartmentation both in the germinal matrices and the intermediate zone. In the neuroepithelium of embryos that survived for 24 h, the differentially labeled cells form two distinct horizontal bands. The band overlapping with the mitotic zone is composed of heavily labeled cells, whereas the band overlapping with the synthetic zone is composed of lightly labeled cells. This indicates that there are two proliferative cell populations within the neuroepithelium, one turning over fast and the other more slowly. In the cortical intermediate zone of the same embryos several horizontal bands are present. Of these, the dispositions of two bands of heavily labeled cells--the superior band and the inferior band--were followed for several days. The superior band is apparently composed of glial cells that disperse in the direction of the internal capsule and the corpus callosum. In contrast, the inferior band (which overlaps with the subventricular zone where many cells are horizontally oriented) is apparently composed of sojourning young neurons. The cells of the inferior band resume their migration toward the cortical plate after a pause of 1-2 days. These observations call for a reappraisal of the view that young cortical neurons follow a direct radial path to the cortical plate.