45 male and female Wistar rats were given a single injection of 3H-thymidine (10µ Ci/g body weight) on day 1, 7, 14 or 21. All animals survived until 60 days of age when they were perfused with 10% neutral formalin and the brains were removed and prepared for autoradiography. The sagittal section of the cortex (L980 µm) was 6.8% larger in the males (p < 0.05) but the packing density of the cortical cells was 5.9% higher in the females (p < 0.01), thus bringing the total number of cells to the male levels. The diameter of the female cortical cells was 3.8% smaller than those of the males (p < 0.05). The greatest difference was among the smaller cells (3–9 µm). The rate of postnatal acquisition of cortical cells was indicated by the number of radioactive-labelled cells. Males had more labeled cells after each injection; it was most pronounced (32% difference) on day 7 (p < 0.05). This may reflect a delayed acquisition rate of cells formed before birth, since more cells could be labeled by the postnatal injec