In red cabbage hypocotyls the photoinduced increase in the activity of the enzyme phenylalanine ammonia-lyase (PAL) is not followed by a decline as found in gherkin hypocotyls. This indicates that a PAL inactivating system as assumed to exist in gherkin hypocotyls does not operate in red cabbage hypocotyls. For gherkin seedlings it has been postulated that at room temperature a slow synthesis of PAL is compensated by continuous inactivation and that at low temperatures the inactivating process is reversed resulting in the release of PAL from an enzyme-inactivator complex. In agreement with the assumption that red cabbage hypocotyls do not have a PAL inactivating system, it has been found that a cold dependent increase in PAL activity does not occur in these hypocotyls and that the rise in PAL activity at room temperature in darkness is much faster than in gherkin hypocotyls. It is suggested that the lack of a PAL inactivating mechanism in red cabbage hypocotyls is compensated by a better control of the photoinduction of PAL. In these seedlings the increase in PAL activity comes to a halt rather abruptly as soon as the light is turned off, whereas in gherkin seedlings in similar conditions the increase may continue for a fairly long time. This goes with a more clear-cut manifestation of phytochrome action in red cabbage hypocotyls as indicated by red/ far-red reversibility and a greater effect of continuous irradiation with far-red light than with other light qualities. The effects of light on the PAL activity in red cabbage hypocotyls are parallelled to a certain degree by those on the accumulation of anthocyanin. Certain discrepancies can be resolved on the assumption that in the synthetic pathway of the latter compound there is at least one other light sensitive step.