Energy transfer from resynthesized protochlorophyllide (PChlide) to chlorophyll (Chl) appeared after 5 h irradiation and additional redarkening of etiolated barley leaves. The transfer was more effective after a longer greening. Benzyladenine (BA) removed delay at the onset of the energy transfer and this process was effective even at very low Chl contents. BA enhanced Chl accumulation more strongly than the development of photosystems. The balance between the rate of Chl synthesis and that of its efflux from PChlide-containing site may determine the number of Chl molecules near PChlide, and thereby the probability of the energy transfer. At a high rate of Chl synthesis a part of its molecules remained near the PChlide synthesis site. Under various conditions of irradiation the slower rate of Chl accumulation corresponded to the more efficient energy transfer. Chl accumulation at PChlide forming site probably Controls the activity of the multienzyme systém of Chl biosynthesis by a feedback mechanism. The BA addition increased the number of EF-mtramembrane particles and the munber of enzyme systems of Chl biosynthesis to the same extent.