Determinations of cell-doubling times using the technique of colchicine-induced metaphase accumulation showed that after 40 h exposure to red or far-red light the rates of cell division in young rice (Oryza sativa L. cv. Ballila) shoot apices were faster than in dark controls. In red light, the increase was already taking place after 16 h of irradiation but in far-red the rates at this time were slower than in the dark controls. Seedlings became more responsive to far-red light as they continued to grow in darkness between 2 1/2 and 4 1/2 days. Mitotic activity at the apex increased in the leaf primordium and decreased in the sub-summit corpus between the 4th and 5th days of growth in darkness at 30° C.