It has been shown that 1-aminocyclopropane-1-carboxylic acid (ACC) is the immediate precursor of ethylene, which is derived from C-2 and C-3 of ACC. When [1-(14)C]ACC was administered to etiolated mungbean (Vigna radiata) hypocotyls, approximately 16% of the ACC was converted to ethylene and about 10% of the radioactivity was converted to [(14)C]asparagine in 7 hr. In etiolated epicotyls of common vetch (Vicia sativa), after 7 hr about 14% of the ACC was converted to ethylene and 16% of the radioactivity was converted to beta-cyanoalanine plus gamma-glutamyl-beta-cyanoalanine. Itis known that in most plants cyanide is metabolized to asparagine via the intermediate beta-cyanoalanine, whereas in a fewplants such as V. sativa, beta-cyanoalanine is converted to the conjugate gamma-glutamyl-beta-cyanoalanine. We confirmed that [(14)C]cyanide was metabolized into [(14)C]asparagine in mungbean and into [(14)C]cyanoalanine plus its conjugate in V. sativa. Moreover, after feeding plant tissue with [1-(14)C]ACC, [(14)C]asparagine isolated from mungbean and beta-[(14)C]cyanoalanine from V. sativa were labeled in the C-4 position, as would be expected if these two compounds were derived from [(14)C]cyanide. When the conversion of ACC to ethylene in V. sativa tissue was inhibited by high temperature (41 degrees C), the conversion of [1-(14)C]ACC to beta-[(14)C]cyanoalanine and gamma-glutamyl-beta-[(14)C]cyanoalanine was similarly inhibited. When [carboxyl-(14)C]ACC was administered to mungbean and V. sativa, (14)CO(2) was recovered in an amount equivalent to the amount of ethylene produced. These data indicate that in the conversion of ACC to ethylene the carboxyl group yields CO(2), and C-1 is released as HCN.