Light-induced loss of the enzyme protochlorophyllide reductase (EC 126.96.36.199.), already described as a characteristic of whole plants, has now been demonstrated in vitro using etioplast membrane preparations of Avena Sativa L. var Peniarth and Secale cereale L. var Rheidol. Some evidence is presented, based upon temperature, pH, and inhibitor sensitivity of the process, that loss of enzyme may be the result of proteolysis. The light-induced process can, in vitro, be largely prevented by addition of the substrates of the reductase, protochlorophyllide and NADPH. It is concluded that light causes the breakdown of the reductase in vivo and in vitro by producing ligand-free enzyme as a consequence of the photoconversion reaction.