The kinetics of extension induced by GA31 in the hypocotyl of intact seedlings of Lactuca sativa are similar in the dark and in the light, and differs fundamentally from the kinetics of elongation in the dark without GA3. Both in continuous light and in the dark, GA3-induced promotion starts 24 h after incubation. In the dark, even low concentrations of GA3, which do not affect the length measured after 6 d when the extension of hypocotyl almost ceases, remove the lag period of 48 h which precedes extension, and prolong the high rate of elongation. Following GA3 supply the hypocotyl length in the dark and in the light does not differ until 48 h; thereafter the rate of elongation in the light is less, so that the final length of the hypocotyl is 40 per cent shorter than that of the dark-grown seedlings without GA3. IAA supplied apically to light-grown seedlings induces a weak promotion at a concentration of 1 mg l−1 only. With an increasing concentration of GA3 supplied simultaneously, the concentration of IAA inducing a significant promotion decreases. A combined supply of both these regulators, however, does not restore the light-mediated inhibition of hypocotyl elongation completely. The maximum decrease in hypocotyl length induced by the growth retardants AMO-1618, CCC, and B-9 supplied from the beginning in the dark does not exceed 70 per cent. Saturating doses of GA3 supplied in combination with any one of the retardants compensate only a fraction of the decrease. The results have been interpreted to show that native GAs are not involved in extension growth in the dark.