The nucleic acids in the green and in the senescent leaves of three types of plant have been studied. High and low molecular weight RNA of the chloroplast is not present in senescent leaves of Xanthium pensylvanicum, but both cytoplasmic and chloroplastic fractions are found in yellow leaves of Vicia faba and Nicotiana tabacum. RNA is more rapidly degraded than DNA in the leaves of these plants when they are detached, and kinetin treatment temporarily arrests the loss of chlorophyll and nucleic acid. Once X. pensylvanicum leaves are yellow and senescent they cannot be re-greened, whereas those of Nicotiana spp., and to some extent those of V. faba, can be rejuvenated. We suggest that the retention of chloroplast RNA in yellow leaves may be a major factor determining their ability to re-green and that the pattern of organelle senescence prior to the first stages of leaf autolysis and dehydration is species-specific.