Rat fetuses received tritiated thymidine on one of each of the following days of gestation: 9, 10, 11, 12, 13, 14 and 16. Animals were sacrificed on day 1 post partum. Perpendicular and tangential sections including temporal cortex were studied by electron microscopy and light microscopic autoradiography. Our results suggest that the formation of Cajal-Retzius cells (CRs) begins on day 12, reaches a peak on day 13, and continues on day 14. Earliest non-CR neurons of the temporal cortex (cells of the deepest stratum of layer VI) appear to be generated on day 13.