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1 In vivo treatment with RNase considerably increases (80%) the RNA content of anucleate and nucleate fragments as well as of whole algae (Acetabularia mediterranea). A parallel increase in the incorporation of phenylalanine into proteins is observed. 2 Prolonged actinomycin (30 \gmg/ml) treatment (7\2-15 days) results in a decrease in the amount of RNA in whole algae and in anucleate apical fragments. This is not, however, the case for the basal anucleate fragments of the stalk. The fact that RNA synthesis in this region resists such treatment with actinomycin seems to bear some relation to the extreme limitation of their morphogenetic capacities. 3 The biological effects of RNase are increased by the simultaneous addition in vivo of actinomycin. 4 Thymus histones and polylysine influence morphogenesis in the same manner as actinomycin though to a lesser extent. 5 Anucleate fragments are much more sensitive than the nucleate fragments to levorphanol, which probably inhibits preferentially ribosomal RNA synthesis. 6 The significance of these results is discussed.