Photoreversible phytochrome disappears from etiolated tissue upon actinic irradiation. Such disappearance, of possible physiological importance, involves several processes, at least one of which is accelerated by metals in vivo. Purified phytochrome from oat (Avena sativa L. cv. Garry) coleoptiles is greatly stabilized in vitro by scrupulous removal of metal impurities via chelating agents. Such stabilized phytochrome decays rapidly upon the addition of about 10 mum Hg(2+), Cd(2+), Cu(2+), and Zn(2+), all of which probably act on sulfhydryl groups. Other tested metals and growth factors were much less active or inactive. The metals effective in promoting decay do not affect the Pfr --> Pr reversion process. This supports other evidence indicating the possible physiological importance of phytochrome "decay."