In leaves of dark grown bean, the degree of reversibility by far red of red-induced induction of rapid chlorophyll accumulation in white light depends upon the duration of a dark period between short red and far red irradiations. Reversibility reaches a maximum for a dark period of about 9 s. It is proposed that this can be explained by interaction of at least two first order processes. The first, with a rate constant at 20°C of 0.23 s~is thought to represent migration of P(r to sensitive sites. The second, with a rate constant of 0.1 s ”1 is the escape from phytochrome control of one of the early steps in the induction process.