Summary The effects of gibberellic acid (GA3) and the putative cotyledon factor dihydroconiferyl alcohol (DCA) on the elongation of hypocotyls of lettuce (Lactuca sativa) were studied. GA3 promotes elongation of the excised hypocotyl of Arctic, Grand Rapids and Great Lakes cultivars of lettuce, and light has a pronounced influence on elongation of these cultivars since it sensitizes the tissue to GA3. Although excised hypocotyls elongate less than intact seedlings in response to applied GA3, this reduced response is not due to removal of the source of a diffusible cotyledon factor since neither the addition of cotyledons nor DCA-treatment restore growth of excised sections. Experiments with inhibitors of DNA-synthesis indicate that the primary effect of excision of cotyledons and apex is to remove the source of new cells that can elongate in response to GA3. Elongation of the hypocotyl of decotylized seedlings is also influenced by GA3 and light in a manner similar to that observed with excised hypocotyls. DCA-treatment does not affect elongation of decotylized seedlings in the presence or absence of GA3, neither does it affect the elongation of intact seedlings under various conditions of incubation. Our evidence does not implicate DCA or any other cotyledon factor in the control of lettuce hypocotyl elongation by GA3.