A short pre-irradiation with red light some hours before the start of continuous illumination shortens the lag phase in chlorophyll-a (Chl-a) accumulation. This induction of rapid Chl-a accumulation was studied in leaves of dark grown seedlings of pea, bean, and maize. Considerable differences in sensitivity to red light were observed; correlation with total spectrophotometrically demonstrable phytochrome was not found. In pea leaf, induction proved extremely light sensitive. The action maximum about 660 nm points to phytochrome as a photoreceptor. It is concluded that phytochrome has a dual effect upon the kinetics of the greening process. Pfr increases the capacity of the biosynthetic system forming protochlorophyllide (Pchlide) and initiates the development of a mechanism which protects chlorophyllous pigments from photodestruction.