The activity of the enzyme nitrate reductase in intact radish seedlings (in vivo activity) was measured in the light (at 200 and at 20,000 lux) and in the dark with 15N labeled nitrate and compared with the activity of enzyme extracts (in vitro activity). Seedlings were illuminated for 24 or for 48 hours. During the first 24 hours the in vivo activity was about the same at the different light intensities whereas the in vitro activity was correlated with the light intensity. After the first 24 hours reduction of nitrate in vivo ceased when at 200 lux or in the dark; in vitro activity, however, was still detectable during the subsequent 24 hours period but at lower levels than for the first period. At 20,000 lux, however, both in vivo activity and in vitro activity increased, although not to the same degree: the induction of the enzyme increased more than the in vivo activity in the seedlings.