The influence of short lasting pre-irradiations from above with monochromatic light on the geotropic reaction of the Avena seedling is dependent on wave length and quantity of the radiation used, on the interval of time inserted between preirradiation and unilateral exposure to gravity (termed “waiting time”), and on the length of the period during which the plants are kept in the horizontal position (termed “exposure time”). With red or far red pre-irradiation merely an increase of the geotropic curvature is observed, but only when a “waiting time” of 30 minutes is inserted and the “exposure time” is about 60 minutes. With blue (or violet) light is observed a decrease in curvature with 1000-10,000 erg cm-2, a “waiting time” of 30 minutes and an “exposure time” ofc. 60 minutes, an increase with 100,000 or more ergs cm~2, a “waiting time” of 0 minutes or of 75 minutes and an “exposure time” of c. 60 minutes. Any preirradiation influences the growth rate of the coleoptile in some way; this applies to plants in the horizontal as well as in the vertical position. Correlations between growth and geotropic reaction are different in different cases. Correlation between phototropic and geotropic behaviour could not be demonstrated. No red – far red antagonism could be detected.