Investigations were conducted with etiolated seedlings of Avena on the influence of red, far red and blue light on: a. the growth of sections of coleoptile and mesocotyl b. the geotropic reactions of coleoptile and mesocotyl c. guttation of the seedlings. The experiments were carried out in darkness, the experimental irradiations excepted. Two systems of reactions to light appear to be present in the seedlings. One of them shows a very low saturation value of about 1 erg cm-2 for red and far red light, the sensitivity to blue light being much lower than for the former wavelengths. This system was found in section growth and the geotropic reaction. The other system shows a dependency on the amount of light energy applied up to at least 105 erg cm-2 for red, far red and blue light. In this system the sensitivity to far red light is much lower than to red light and about as high as to blue light. This system was found to affect guttation and the shape of the geotropic curves of the seedlings. The latter system appears to exert its influence through changes in transport rates of water and solutes, as is indicated by its influence on guttation. From the results is concluded that this change in the rate of water transport influences the transport rates of auxins and, consequently, the relative growth rates of different parts of the seedlings. Connections of the conclusions drawn with some data from the literature on phototropism are discussed.