AbstractThe effect of red and far‐red treatment on chlorophyll synthesis in dark‐grown bean leaves was studied at various ages. Although the effect was pronounced in the old leaves, no effect was observed in the young ones (4 days old). In the 5‐day old leaves a measurable effect of red light pretreatment can be observed, whereas the far‐red reversal effect was not observed. — The length of the dark period between the red pretreatment and the continuous illumination is also age dependent. Leaves older than 6 days show a maximum at about six hours, while in the young leaves the red light effect increases with the time of dark incubation up to the 24 hours tested. — The reversal effect of far‐red light on protochlorophyllide regeneration was also examined. The far‐red light has no reversal effect on leaves younger than 6 days old, while on the old leaves it has