1. Chloroform in low concentration (0.25 per cent) causes an increase in the rate of production of CO2 in Ulva; this is followed by a decrease. In higher concentration (0.5 per cent) only a decrease is observed. 2. Assuming that the normal oxidation depends on the action of peroxide and peroxidase, experiments were made by placing Ulva in 1.0 per cent H2O2 and in Fe2(SO4)3 (which acts like a peroxidase). The former diminishes the rate, the latter increases and subsequently decreases it. 3. When Ulva is killed in such a manner as to destroy the oxidizing enzymes, no CO2 is produced unless H2O2 and Fe2(SO4)3 are present. If to this mixture chloroform is added, the effect depends on the concentration of the iron. If the concentration is low there is an increase in the production of CO2 followed by a decrease. If the concentration is high the rate appears to decrease from the start.