Determinations of the catalase activity of 20-25 mgm. of fresh apical bud tissue of spinach plants, taken 2.5, 16.5, and 40.5 hrs. after a change to a longer photoperiod, show that the plants have responded to the stimulus by an increase in the catalase activity, indicating that spinach has responded rapidly to the change in day length. The Virginia Savoy var., which produces seed-stalks more quickly than the Old Dominion under a favorable photoperiod, has shown a greater change in the catalase activity.