In etiolated hypocotyls of Raphanus sativus L. the growth responses to continuous red, far-red and blue light have been distinguished on the bases of photoreceptive sites and regions of physiological response. Blue light appeared to retard a fairly mature stage of elongation, acting immediately and directly on the cells irradiated. Far-red light caused a marked inhibition of all stages of elongation after a lag period, and the stimulus could be transmitted from the hook region. The effect of red light was complex and consisted of one promotive and two inhibitory responses.