1. With the aim of elucidating the path of carbon in the formation of ethylene in plants, studies were made on the incorporation of 14C into ethylene evolved from apple slices, using several 14C- labeled compounds as substrates. The effects of inhibitors were also investigated. 2. The formation of ethylene-14C from glucose-14C was inhibited by fluoride, but unaffected by arsenite, thus suggesting that glucose is converted to ethylene via pyruvate. 3. Acetate is converted to ethylene after cleavage of C-l and C-2. Only a small portion of the latter (C-2) enters the molecule of ethylene, the former (C-l) is detected in carbon dioxide. On the other hand, 2, and 3-carbons of pyruvate are converted, without splitting, to ethylene. 4. On removal of air, the incorporation of 14C into ethylene from acetate-2-14C was depressed, while that from pyruvate-14C was unaffected. 5. Acetaldehyde-l,2-14C is converted to ethylene without conversion into ethanol. 6. These results are interpreted to suggest the occurrence of the pathway in which pyruvate and acetaldehyde may serve as precursors of ethylene.