During the illumination of etiolated barley plants a rapid decline of the NADPH: protochlorophyllide oxidoreductase is observed. Within the first 5 min of continuous light approximately 90% of the enzyme activity present in dark-grown barley plants disappears and, at the same time, the amount of enzyme protein is diminished by more than 60%. No stable polypeptide fragments have been found which might be formed during the light-induced degradation of the enzyme protein. The rate of enzyme protein synthesis is not drastically affected at the beginning of the illumination period. During the subsequent light-dependent chloroplast development a phytochrome-induced decline in the rate of protein synthesis, concomittant with a continuous light-dependent degradation of the enzyme protein, leads to a progressive decrease of the concentration of the enzyme. After 6 h of continuous light, when the rate of chlorophyll accumulation is at its greatest, only traces of the enzyme protein are visible and the enzyme activity is no longer detectable within the plants. In contrast to previous concepts of chlorophyll biosynthesis in higher plants, our results present evidence that the NADPH: protochlorophyllide oxidoreductase functions only for a short time period after the onset of light.