The effects of fusicoccin (FC) and gibberellic acid (GA 3) on the growth of lettuce hypocotyls were studied. FC promotes rapid elongation of excised hypocotyl sections with a lag time of 10 min. The initial rate of FC-induced elongation is 10–20 times higher than growth in H 2O and 3–4 times higher than growth with GA 3. FC-induced elongation declines exponentially with time, so that by 8 h of incubation, growth with FC is at the same rate as growth in H 2O. Growth in FC is manifested by an increase in section width as well as section length. Growth in both width and length occurs in the top, middle and bottom regions of FC-treated sections, while after GA 3-treatment sections grow only in length and predominantly in the upper region of the hypocotyl. Although FC promotes radial expansion it does not affect tubulin polymerization in vitro, suggesting that it might not affect the assembly of cytoplasmic microtubules. We propose that the mechanisms underlying GA 3- and FC-induced growth are different.