WHEN ACTINOMYCIN D, PUROMYCIN, STREPTOMYCIN, CHLORAMPHENICOL, AND CYCLOHEXIMIDE, KNOWN INHIBITORS OF PROTEIN SYNTHESIS, WERE APPLIED TO LEAVES OF INTACT SEEDLINGS OR DETACHED LEAVES OF BARLEY PRIOR TO THEIR GREENING, THE SAME GENERAL RESPONSE RESULTED: the light-induced increase in activity of ribulose 1,5-diphosphate carboxylase was prevented while that of phosphoribulokinase was only partially suppressed; synthesis of chlorophyll was arrested. This is taken as preliminary evidence that de novo synthesis of protein may be responsible for the observed increase in ribulose-1,5-diphosphate carboxylase activity during greening. However, other factors may be involved with the light-induced stimulation of phosphoribulokinase.Carbohydrate metabolites and substrates of the enzymes failed to induce the formation of ribulose-1,5-diphosphate carboxylase and phosphoribulokinase in the dark. No evidence was found for the presence of inhibitors in etiolated seedlings or activators in illuminated leaves of barley. Carboxylase activity almost equal to that of the illuminated water control was stimulated by MgCl(2) in the dark; MgCl(2) had no effect on the activity of the kinase.