In 1934, Du Buy and Nuernbergk (9) compiled from a number of sources the first extensive phototropic dosage-response curve for oat coleoptiles. The various features of this curve, first positive curvature, first negative curvature, and second and third positive curvature, are now well known. The early dlosage-response work in Avena was not done under controlled condition of light intensity, spectral purity of the light source, temperature, humidity, and growth regime. Hence many conflicting results were published. Intensities were usually measured in ft-candles or similar units, rather than in absolute units such as quanta per cm2 per second at a particular wavelength. Combinations of intensity and length of exposure used to achieve a particular light dosage have rarely been reported. The importance of precise knowledge of light exposure conditions in interpreting dosage-response curves has been emphasized elsewhere (4) and will be considered in detail in the discussion. Only very recently has the profound effect of red light on the phototropic response been appreciated (1, 3, 5, 7, 8). But here too the picture was confused. Some authors reported an increase in phototropic sensitivity (1, 3, 5) while others reported a decrease following exposure to red light (1, 7, 8). The manner in which red light was administered varied widely from one study to the next and it is therefore very difficult to evaluate these results or draw any valid conclusions from them. The dosage-response work of Curry (7) is the first that may be considered reliable. The plants in his experiments were all given red light prior to phototropic induction with light of wavelength 4358 A. In general, however, Curry worked with high intensities and short exposure times, without specifying them in every case, so that in spite of his use of a calibrated monochromatic source, the picture he obtained was incomplete. Under these conditions, first positive and first negative curvatures were obtained as well as an additional positive component occurring at very high dosages. Curry, however, designated the latter as base curvature. Thus, it is not clear that this can be equated with the second positive curvature of the present study which is strictly a tip response. Thimann and Curry (11) have extracted approximate dosage-response curves at various intensities from the data of many workers. Thus they have clarified the confusing dosage-response picture somewhat, and have drawn attention to the fact that it is not simply the total amount of light received by the plant that is important, but the manner in whiclh it is a(lministered as well The present paper has three objectives. The first is to obtain a series of dosage-response curves for phototropic tip curvature with several intensities of monochromatic light and exactly defined conditioiis. The second is to clarify the effect of red light on phototropic sensitivity under a wide range of defined conditions of phototropic induction. The third is to provide the basis for a detailed kinetic analysis of phototropism of oat coleoptiles (13).