The role played by the nucleus in the circadian rhythms in Acetabularia mediterranea was investigated: these rhythms were in photosynthetic capacity and in chloroplast shape. The following results were obtained. 1. 1. Anucleate Acetabularia are able to maintain their rhythm of photosynthetvc capacity and to restore this rhythm after it has been abolished by physical means. 2. 2. The same conclusion is true for the rhythm in chloroplast shape. 3. 3. Actinomycin D dramatically inhibits the rhythm in photosynthetic capacity in intact algae. The rhythm is strongly decreased within 6 days by a concentration of 2.7 μg/ml; at a concentration of 0.27 μg/ml for 12–16 days the rhythm is abolished or very strongly reduced, depending on the experiment. 4. 4. Actinomycin D also prevents the rhythmic variation in chloroplast shape with the time period of the cycle, at the same concentration and for the same length of treatment as used for the inhibition of the rhythm in photosynthetic capacity. 5. 5. Actinomycin D does not affect the rhythm in photosynthetic capacity in anucleate algae at the concentrations used in the experiments with intact Acetabularia. 6. 6. No difference could be detected in the behaviour of anucleate basal fragments and anucleate apical fragments in the presence of actinomycin D. 7. 7. Actinomycin D does not affect the rhythm of variation in shape in chloroplasts in anucleate algae. 8. 8. The two circadian rhythms, affecting photosynthetic capacity and chloroplast shape, were always correlated. 9. 9. Ribonuclease induces an increase in the rhythm of photosynthetic capacity when the nucleated algae are returned to normal sea-water; on the contrary, the rhythm is not restored with photosynthesis when the treated algae are anucleate. The results are discussed on a molecular basis and a tentative interpretation is proposed, compatible with the present experiments and with results in the literature.