The incorporation of 14C-leucine into senescing disks from corn leaves was measured in the presence and absence of 6-benzylaminopurine (benzyladenine). Differences in the amount of incorporation were detected as early as 90 min after start of the hormone treatment. Tissue treated for 24 hr with benzyladenine incorporated up to twice as much 14C-leucine as untreated tissue. This difference may reflect an effect of the hormone on the size of the protein precursor amino acid pool resulting in differences in the isotopic dilution of the added 14C-leucine, rather than an effect of benzyladenine on the rate of protein synthesis. Leaf disks pre-labeled with 14C-leucine for 4–5 hr and subsequently transferred to media containing unlabeled leucine with or without benzyladenine showed a slower breakdown of the pre-labeled protein in the presence of the cytokinin. Since the specific activity of the protein of leaf disks incubated with or without benzyladenine did not change over 48 hr, it is suggested that cytokinins retard senescence of corn leaves primarily through inhibiting protein degradation.