The red/far-red system has a short-term action on basic peroxidases in spinach leaves (Spinacia oleracea). 2 min of far-red changes their activity. Red light reverses this effect. The nature of the response depends on the energy given to the plant at 730 nm. In plants exposed to short days and in the same energetic conditions this photosensitive reaction follows a daily rhythm. When these plants are transferred under continuous light, the rhythm disappears. After a 48-h light period, the plants exposed to a long night (16 h) do not recover their short day response. Continuous light seems to induce a new sensitivity of basic peroxidases to light.