1. Light-induced oxidation of cytochrome f in greening bean leaves was first observed after 90 min of illumination. Intense red light was needed to drive the oxidation. At 2 h cytochrome f oxidation was driven by 675 nm monochromatic light, but not by 703 nm light. By 2.5 h the oxidation was driven by 703 nm light and now 675 nm light was less effective. Both the extent and the initial rate of cytochrome f oxidation in 675 nm light were increased more than 10-fold by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). These results show that both Photosystems I and II are active at 2.5 h, which correlates with the end of the lag phase in chlorophyll synthesis. Photosystem I is active ahead of Photosystem II. 2. If etiolated leaves are photoconverted and then returned to darkness for at least 2.5 h, Photosystem I is active at the second photoconversion and Photosystem II develops within a period of minutes. In the presence of DCMU, actinic light of 688 nm is more effective than 667 nm or 675 nm light in photooxidizing cytochrome f, even though the absorption maximum of the leaf is at 672 nm. 3. Photosystem II activity, as measured by the ability of 675 nm light to hold cytochrome f mainly in the reduced state was not dependent on the presence of grana or cytochrome b-559.