Phytochrome has been measured in etiolated seedling of Amaranthus caudatus. The phytochrome content increases from the time of germination until 72 hr from sowing, after which it remains constant at 27.5x10(-3) Δ (ΔOD) units per 200 seedlings. After a saturating dose of red light P fr decays in the dark to a form not detectable photometrically. There is no evidence for the process of dark reversion of P fr to P fr found in other dicotyledons. Even in the presence of azide, a selective inhibitor of decay, the process of dark reversion is not observed. The decay of P fr has been investigated at different temperatures and follows first order decay kinetics throughout. Over the temperature range 15-30° the Q 10 of decay remained constant at 4.3.The photostationary states of phytochrome (P fr /P total )maintained by mixed red/far-red light have been measured in both seedlings and partially purified protein extracts, with good agreement. The rate of phytochrome decay can be manipulated by changing the P fr /P total ratio. The lag period before a decay curve becomes exponential is characteristic of a particular P fr /P total ratio and represents the time for attainment of the photostationary state. The effect of energy on decay has been investigated under red and blue light. The rate of phytochrome decay is dependent on the P fr /P total ratio and only becomes energy dependent when the light intensity is so low that the photostationary state is never attained.The process of apparent phytochrome synthesis has been found in Amaranthus. After reducing the phytochrome to a low level by red light treatment a rate of apparent synthesis of 1.35×10(-4) Δ (δOD) units per hr per 200 seedlings was observed, levelling off at 29% of the original phytochrome level.Under white tungsten lights of high intensity there is a deviation from the expected first order decay kinetics. The nature of this low rate of decay cannot be explained at the present time.