Abstract 1. 1. There is an increase in the extractable activity of phenylalanine ammonia-lyase following excision of hypocotyl segments of dark-grown gherkin seedlings. This increase is prevented by the reaction product, cinnamic acid. 2. 2. Density-labelling experiments show that cinnamic acid affects the rate of enzyme synthesis. By contrast the regulation of phenylalanine ammonia-lyase activity by light has previously been shownto involve activation of existing inactive enzyme. 3. 3. It is proposed that regulation of synthesis by reaction products represents a mechanism for controlling the size of the pool of phenylalanine ammonia-lyase, and that the activity of this pool is regulated by light.