NSC 16590 (α-Aminoisobutyric acid , AIB) inhibits the production of endogenous ethylene in the cotyledonary segments of cocklebur ( Xanthium pennsylvanicum Wallr.) seeds most strongly. NSC 16590 at 4 mM inhibits the formation of ethylene by about 50%, although the O ₂ uptake of the segments is not affected even at 20 mM. NSC 16590 also inhibits ethylene formation in the stem segments of etiolated pea ( Pisum satiuum L. cv . Alaska) seedlings. Kinetic analysis with cell free extracts from etiolated pea shoots reveals that NSC 16590 competitively inhibits the conversion of ACC into ethylene.