1. Density-labelling with 99 atoms% of (2)H(2)O distinguished pre-existing from newly synthesized ribonuclease molecules in sections of developing hypocotyl tissue. 2. Activity profiles of enzyme extracted from the fraction pelletable at 100000g showed heterogeneity after isopycnic centrifugation in CsCl gradients. 3. Measurement of density shifts of the entire heterogeneous band shows that ribonuclease protein is synthesized de novo in both continuous far-red light and darkness. 4. A twofold increase in enzyme activity after irradiation was accompanied by band-broadening and a significantly faster rate of labelling than in darkness. 5. The conclusion is drawn from the experimental evidence and theoretical arguments presented that phytochrome regulates the synthesis of new enzyme molecules against a background of continuous (dark-rate) synthesis and degradation. 6. Further information has been deposited as Supplementary Publication SUP 50033 (3 pages) at the British Library Lending Division (formerly the National Lending Library for Science and Technology), Boston Spa, Yorks. LS23 7BQ, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1973), 131, 5.