The speed of ethylene-induced leaf abscission in cotton (Gossypium hirsutumL. cv LG-102) seedlings is dependent on leaf position (i.e.physiological age). Fumigation of intact seedlings for 18 hours with 10 microliters per liter of ethylene resulted in 40% abscission of the still-expanding third true (3°) leaves but had no effect on the fully expanded first true (1°) leaves. After 42 hours of fumigation with 50 microliters per liter of ethylene, total abscission of the 3°leaves occurred while<50% abscission of the 1°leaves was observed. On a leaf basis, endogenous levels of free IAA in 1°leaves were approximately twice those of 3°leaves. Free IAA levels were reduced equally (approximately 55%) in both leaf types after 18 hours of ethylene (10 microliters per liter) treatment. Ethylene treatment of intact seedlings inhibited the basipetal movement of [14C]IAA in petiole segments isolated from both leaf types in a dose-dependent manner. The auxin transport inhibitorN-1-naphthylphthalamic acid increased the rate and extent of ethylene-induced leaf abscission at both leaf positions but did not alter the relative pattern of abscission. Abscission-zone explants prepared from 3°leaves abscised faster than 1°leaf explants when exposed to ethylene. Ethyleneinduced abscission of 3°explants was not appreciably inhibited by exogenous IAA while 1°explants exhibited a pronounced and protracted inhibition. The synthetic auxins 2,4-D and 1-naphthaleneacetic acid completely inhibited ethylene-induced abscission of both 1°and 3°explants for 40 hours. It is proposed that the differential abscission response of cotton seedling leaves is primarily a result of the limited abscission-inhibiting effects of IAA in the abscission zone of the you