Red light enhances the geotropic response of excised Avena coleoptiles. With higher dosages this enhancement occurs in the main in the upper centimeter of the organ and is compensated by a decrease in the geotropic response of the base. In both of these aspects the red light induced alterations are dosage dependent in the range of irradiancies investigated (from 2000 to 240 000 erg/cm2; intensity: 1000 erg/cm2 sec). The influence of even a relatively small dosage of red light (2000 erg/cm2) is evident for at least four hours after the illumination. Possible relations with phototropic phenomena are discussed. From the current hypotheses about the mechanism of the red light-induced alterations in tropic responses, that of red light influencing the pigment system active in phototropic perception is considered to be the least attractive one.