The use of growth substances upon plants as herbicides and for the purpose of modifying such phenomena as dormancy, abscission, bud differentiation, and fruit set has become quite extensive. Therefore, it should be of interest to know what effect some of these compounds have upon the process of photosynthesis. Very few data have appeared in the literature in regard to this process in relation to growth substances. Mitchell, Kraus, and Whitehead (5) report that the leaves of bean plants which had been depleted of sugar, starch and dextrin, by being kept in the dark, accumulated less sugar, starch and dextrin during subsequent illumination when they were treated with alpha-naphthalene-acetic acid than similar untreated control plants. The authors suggest that one possible explanation of these data may be a reduction in the rate of photosynthesis. In the present problem in which apparent photosynthesis was measured it seemed desirable to record the rates of respiration as one basis for interpretation of the results. Pratt (?) found that indole-3-acetic acid at 0.1 to 100 p.p.m. increased respiration of wheat seedlings but that higher concentrations of the auxin decreased respiration. Berger, Smith and Avery (1) concluded that indole-acetic acid at 10 mg. per liter stimulated respiration in Avena coleptiles. Zimmerman and Hitchcock (8) reported that naphthalene-acetic acid caused a decrease in the rate of respiration of tomato plants during the first hour after treatment after which the rate was faster in the treated plants than in the controls. Hsueh and Lou (4) reported that 2,4-D at a concentration of 0.1 per cent, inhibited aerobic respiration in barley and rice. The amount of oxygen used was greatly decreased as a result of the treatment. The production of C02 following treatment with 2,4-D was accelerated at first and then decreased but not as much as oxygen utilization. In terms of C02 production Brow^t (2) found that treatment with 2,4-D at 0.1 per cent, concentration resulted in a higher rate of respiration in bean plants. The results of Taylor (7) indicate that 2,4-D at concentrations 0.25 to 10 p.p.m. decreased respiration in wheat and mustard seedlings. Methods