1. Exposure of dark-grown gherkin seedlings to red or far-red light causes changes in the level of phenylalanine deaminase (PADAse) which follow the same time course and show a similar dependence on light intensity as the changes induced in blue light. From this it is concluded that light of different regions of the visible spectrum is initiating the same sequence of dark reactions leading to enhanced production of PADAse, followed by a phase of inactivation of this enzyme and repression of its synthesis. 2. In blue light a higher PADAse level can be obtained than in red or far-red light, and only blue light is capable of inducing PADAse synthesis in excised hypocotyls. This suggests that different pigment systems are involved in the high-energy phenomena in the gherkin seedling. 3. The effects of irradiation sequences with different light qualities and intensities on the accumulation of hydroxycinnamic acids can be explained on the basis of the regulatory mechanisms which control of PADAse level.