1. Orange light, as used in the air-conditioned dark rooms, influences the reactions of Avena coleoptiles to indole-3-acetic acid (I.A.A.). 2. The orange light does not act upon the coleoptile directly, but through the intermediary of the tip of the primary leaf. 3. Light of A = 660 m/i as well as light of A = 740 m/t appeared to be active. 4. Irradiation of the Avena seedling as a whole, with 700 ergs/cm2 A = 660 m/i has two effects: a. it reduces the I.A.A. content of the coleoptiles. b. it enhances either the amount or the effect of an unidentified substance, here called the red light factor (R.L.F.). 5. The R.L.F. regulates the growth of the coleoptile in a narrow range of I.A.A. concentrations. It enhances the growth rate at low I.A.A. concentrations. In this way the one-peaked curve which presents growth as a function of the I.A.A. concentration is transformed into a two-peaked one. 6. Irradiation with 6000 ergs/cm2 A = 740 mju annihilates effect 4b of light of A = 660 m,«, but reduces, like the latter, the I.A.A. content of the coleoptile. 7. An explanation of the negative and of the second positive curvature could be based on a further study of the interaction of the R.L.F. with I.A.A. 8. The results of our experiments are a warning that the influence of orange and red light on the results of growth-substances tests should not be underestimated-