Red light treatment of etiolated plants induces remarkable morphological changes. About the biochemical pathways leading to these effects little is known as yet. It seems logical, however, to suppose a relationship between changes induced in the chemical composition of the plant directly after red light treatment and the morphological changes found later on. Thus, in pea plants the red-light-dependent compound quercetin-3-p-coumaroyltriglucoside has been suggested to be a possible growth mediator by an effect on indoleacetic acid oxidase (Bottomly c.s. 1966). From my work on the biochemical aspects of red light treatment of 7-day old pea plants another red-light-dependent compound emerged.