1. The changes of the rate of elongation of the stem of dark grown Vicia faba seedlings, after illumination by white light, have been measured by auxanometer. 2. Illumination of the plumular hook region, including the apex, causes a reduction in the extension rate of the tissue below. This has been called the ‘primary phase’, and it is complete within three hours of illumination. In the range of exposure used, the new rate may be between 50 and 20 per cent, of the original rate. 3. Illumination of the extending tissue alone causes a similar primary phase, which is followed by a period of acceleration during the third hour after illumination. This period has been called the ‘reaction phase’, and it reaches its peak rate, which is of the order of 60 per cent. of the original rate before treatment, about 4 hours after illumination. 4. Illumination of the whole plumule causes a primary phase followed by a reaction phase of more variable form and size. 5. The magnitude of the primary phase appears to increase with the exposure. There is an optimum exposure for the production of the reaction phase in the region of 200 foot-candle seconds. 6. The significance of these results in the interpretation of etiolation phenomena is discussed.