Hai H o n g Zhong,all A n d r e w S. Resnick,a,2 M a r t i n Straume,b a n d C. R o b e r t s o n M~C l u n g~.~ a Department of Biological Sciences, Dartmouth College, 6044 Gilman Laboratory, Hanover, New Hampshire 03755 and Metabolism, University of Virginia, Charlottesville, Virginia 22903 National Science Foundation Center for Biological Timing, Department of Interna1 Medicine, Division of Endocrinology Persistent oscillation in constant conditions is a defining characteristic of circadian rhythms. However, in plants transferred into extended dark conditions, circadian rhythms in mRNA abundance commonly damp in amplitude over two or three cycles to a steady state level of relatively constant, low mRNA abundance. In Arabidopsis, catalase CAT3 mRNA oscillations damp rapidly in extended dark conditions, but unlike catalase CAT2 and the chlorophyll a/b binding protein gene CAB, in which the circadian oscillations damp to low steady state mRNA abundance, CAT3 mRNA oscillations damp to high steady state levels of mRNA abundance. Mutational disruption of either phytochrome-or cryptochromemediated light perception prevents damping of the oscillations in CAT3 mRNA abundance and reveals strong circadian oscillations that persist for multiple cycles in extended dark conditions. Damping of CAT3 mRNA oscillations specifically requires phytochrome A but not phytochrome B and also requires the cryptochromel blue light receptor. Therefore, we conclude that synergistic signaling mediated through both phytochrome A and cryptochromel is required for damping of circadian CAT3 mRNA oscillations in extended dark conditions.