The senescence of excised primary leaves of Phaseolus vulgaris L. was followed by measuring the net breakdown of chlorophyll, protein, RNA, proteinase and ribonuclease activity. A decline of chlorophyll, protein, and RNA mark the progress of senescence in excised leaves. The chemical growth regulators, indoleacetic acid and gibberellic acid, were relatively ineffective in retarding senescense. N5-Benxyladenine at the concentration of 1 x 10^(-5)M was effective in retarding the decline in the amount of chlorophyll, protein and RNA. In the dark, L-serine at the concentration of 3 x 10^(-2)M promoted yellowing and RNA degradation of excised Ieaves. But in the light, it was ineffective in promoting senescence. Fluorescent light of 1,000 Lux, was very effetive in senecence retardation. In the dark, adding sucrose (5.8 x 10^(-2)M) to the culture medium replaced the light function in retarding the chlorophyll, protein by cycloheximide (5 μg/ml). It is assumed that, upon excision of leaves, the synthesis of some m-RNA responsible for synthesis of destructive enzymes is activated, which ultimately Ieads to the breakdown of RNA and the protein synthesizing machinery. The protein inhibitors could then be expected to temporarily retard chlorophyll breakdown primarily by inhibiting the synthesis of destructive enzymes. The role of light and N5-Benzyladenine in the retardation of senescence may act synchrononously.