1. Disks of tobacco leaf were incubated on water in darkness at 25°. Diagonal cutting of disks had no significant effects on chlorophyll decrease, α-amino nitrogen increase or acid phosphatase increase. Cutting slightly stimulated protein decrease, stimulated peroxidase increase and inhibited catalase decrease. The central areas of larger disks only differed from the control disks in decreased chlorophyll disappearance and peroxidase synthesis. 2. Peroxidase extracts showed some latent activity when treated with Triton X-100, 2,4-dichlorophenol or frozen and thawed. This was insufficient to account for the marked stimulation of synthesis in senescing disks. There was no evidence for cytoplasmic inhibitors of peroxidase. 3. Biochemical differences between chlorotic and green areas of detached, senescing leaves were compared with differences between freshly cut (green) and senescing (chlorotic) leaf disks. 4. Studies with actinomycin D, chloramphenicol, puromycin and actidione suggest that these compounds inhibit the synthesis of some proteins in senescing disks more effectively than others. 5. The differences between disks cut from mature and young leaves were studied.