Kinetin and a,a'-dipyridyl prevented the rapid decrease of chlorophyll content in detached oat leaves senescing in the dark. In the light, detachment caused a 27-40% rise in peroxidase activity and kinetin enhanced the enzyme in the segments by about 80%. Darkness prevented any detachment-induced rise of the activity and decreased the stimulating action of kinetin and mechanical injury. The effect of dipyridyl on peroxidase activity in the dark was similar to that of kinetin. Kinetin enhanced the same distinctive isoperoxidases under light and dark conditions. Neither horseradish peroxidase nor that extracted from oat leaves showed any ability to hydroxy late free proline in vitro. A system which supposedly led to peroxidase-catalysed proline hydroxy lation yielded small amounts of hydroxyproline in the absence of the enzyme. Staining with Fast Blue BB salt in the presence of IAA as a substrate after electrophoresis indi cated that all detected oat isoperoxidases had an IAA oxidase activity visually paralleling their per oxidase activity. Crude extracts contained IAA oxidase inhibitors that could be partially or fully removed by dialysis. The possible significance of the rise in peroxidase activity during senescence is discussed.