In Rana pipiens tadpoles, thyroxine (T 4) injection in the early light promotes faster hindlimb growth and development than injection early in the dark. T 4 also stimulates a significant increase in cell proliferation in the basal epidermal cells of the limb. Using autoradiography, we studied the timing of the T 4-induced rise in the labeling and mitotic indices on 12L:12D cycles with 0.2 μg T 4 injection early in the light or dark and on a 12L:3D:1L:8D cycle with a light pulse early in the dark shortly after a T 4 injection. In all instances, the labeling index peaked at the same time after the start of T 4, so diurnal differences in the binding or initial actions of T 4 leading up to the entrance of epidermal cells into the DNA synthetic (S) phase of the cell cycle are not indicated. However, with T 4 injection in the dark the subsequent mitotic index peak was delayed, to a greater extent on 12L:12D than on 12L:3D:1L:8D. Cells induced to proliferate following T 4 injection in the dark evidently had longer cell cycles, probably at the expense of the S or G 2 phase, than when T 4 was given in the light.