The effects of red, far-red, and blue light on the geotropic response of excised coleoptiles of Zea mays have been investigated. Seedlings were grown in darkness for 5 or 6 days, exposed to various light treatments, and then returned to darkness for determination of the geotropic response. The rate of response of the coleoptiles is decreased after they have been exposed to red light (620–700 mμ, 560 ergs cm−2sec−1 for the 24 hrs, but not for the 4 hrs, preceding stimulation by gravity. Furthermore, their rate of response is greatly reduced if they are exposed to red light for 10 min and then returned to darkness for 20 hrs before geotropic stimulation. At 25° C an interval of 6 to 8 hrs elapses between a 10-min exposure to red light and the first detectable decrease in the geotropic response of the coleoptile. This interval can be lengthened by exposing the seedlings to low temperatures (0° to 2° C) after the light treatment but cannot be greatly shortened by increasing the duration of exposure to red light. Using a standard procedure of exposing 5-day-old etiolated seedlings to light for various times, replacing them in darkness for 20 hrs and then determining the response of the coleoptiles to 4 hrs geotropic stimulation, it has been found that: (a) Exposure to red light for 15 sec significantly decreases the geotropic curvature of the coleoptiles and that further reduction occurs on increasing the length of the light treatment to 2 and 5 min. (b) Far-red light has no effect on the geotropic response of the coleoptiles but it can completely reverse the effect of red light. After repeated alternate exposure to red and far-red light the geotropic response of the coleoptile is determined by the nature of the last exposure, (c) Complete reversal of the effect of red light by far-red radiation only occurs when exposure to far-red follows immediately after exposure to red. The reversing effect of far-red radiation is reduced if a period of darkness intervenes between the red and far-red light treatments, and is lost after a dark interval of approximately 2 hrs. The effect of red light on the rate of geotropic response of the coleoptiles is independent of their age and length at the time of excision. Blue light acts in a similar way to red light, but the seedlings are less sensitive to blue than to red light. Coleoptiles grown throughout in a mixture of continuous, weak, red, and far-red light have a lower rate of geotropic response than etiolated coleoptiles.