1. The temperature dependence of each of the three phases (lag phase, increase of the enzyme level, decrease of this level) of the photoinduced development of phenylalanine deaminase (PADAse) in gherkin seedlings has been determined. 2. For the temperature range of 12-32°, the higher efficiency of the irradiation at a higher temperature is compensated by a shorter duration of the period of light sensitivity, whereas the higher rate of PADAse synthesis is compensated by the earlier onset of repression of PADAse synthesis and of inactivation of previously formed enzyme. 3. At 10° the photoinduced increase of PADAse is no longer followed by a decline. PADAse synthesis seems to be discontinued at about 8°; irradiation of the seedlings at this temperature does not lead to the building up of a potential causing PADAse synthesis at a higher temperature. 4. A cold treatment (4°) decreases the light sensitivity of the gherkin seedling. A period of low temperature following a light treatment diminishes the effect of the latter.