Cell proliferation was studied in the developing mouse retina by autoradiography following injection of [3H]thymidine. In 1-day-old mice, the duration of the phases of the cell-division cycle was investigated in ventricular cells, the mitotically active precursors of postmitotic cells that comprise the neural retina. In the center of the retina, the generation time (T) was 30 h, consisting of 8.5 h G1 phase, 16.0 h S phase, 2.6 h (minimum) G2 phase, and 2.5 h mitosis. In the periphery, T was 28.5 h; G1, 7.5 h; S, 16.0 h; minimum G2, 2.6 h; and mitosis, 2 h. As embryogenesis proceeds, the mitotic cycle decelerates. In contrast with postembryonic cell populations (in which G2, S and M tend to be invariable), all phases of the cycle increase in duration, and most become longer than is commonly observed in mature animals. Approx. 18 generations of ventricular cells are produced. However, throughout the stage of multiplication, some cells cease dividing and differentiate. In 1-day-old mice, 23% of ventricular cells in the center of the retina and 37% in the periphery remain mitotically active. DNA synthesis ceases on the 6th day in the center and the 11th day in the periphery.