The enzymatic activity of a hormone-stimulated and growth-related protein disulfide-thiol oxidoreductase (NADH oxidase) with protein disulfide-thiol interchange activity of etiolated hypocotyls of soybean oscillates with a period of about 24 min or 60 times per 24 h day. The oscillations were temperature compensated such that the period remained constant at about 24 min between 17 and 37°C, a temperature range over which enzymatic activity varied approximately fourfold (Q10 of 2). The oscillations were observed with intact tissue sections, with isolated plasma membrane vesicles, and with the detergent-solubilized and partially purified enzyme. The oscillations were observed both with the oxidation of NADH and in the restoration of activity to scrambled ribonuclease used as a measure of the protein disulfide-thiol interchange activity. The enzymatic activity which is located at the cell surface may represent a biochemical reaction with a potential function as an ultradian oscillator of circadian time keeping.