The concept of hormonal regulation of plant growth, gradually developed during the first quarter of this century, eventually led to the Cholodny-Went theory of tropistic movements. According to this theory the curving response of stems and roots to unilateral photo-and geotropic stimuli results from an uneven lateral distribution of auxin. It was earlier reported (2) that the evidence in favor of the Cholodny-Went theory of phototropism is only indirect, and that physicochemical analysis of the distribution of endogenous indoleacetic acid (IAA) in the phototropically curving hypocotyl of green sunflower seedlings could not establish any lateral auxin gradient. As an alternative, an inhibitor gradient was suggested, with xanthoxin as a possible candidate, analogous to the probable function of abscisic acid in the geotropism of roots. The present paper provides further evidence for the view that phototropism is a phenomenon of growth inhibition, of course requiring a growing tissue to take place, but independent of the rate of elongation of that tissue.