Abstract 1. 1. A diurnal rhythm in DNA synthesis, as indicated by fluctuations in total 3H thymidine uptake, was demonstrated in the corneas of frogs ( Rana pipiens). exposed to a 12 : 12 light-dark lighting régime for 2 weeks and then sacrificed in groups of twelve at 2-hr intervals over a 24-hr period. 2. 2. This rhythm was reproducible, appeared to have a periodicity of nearly 24 hr and was correlated with a similar rhythm in mitotic activity in corneas. 3. 3. Evidence is presented which suggests that the lighting régime may be involved in the regulation of this rhythm in DNA synthesis.