In adult males of the giant silk moth Hyalophora cecropia (L.) the amount of juvenile hormone (JH) extractable from the abdomens increases sharply between the first and fourth day of adult life; 4-8 day-old moths contain up to 6 μg. During the biosynthesis, L-methionine provides the ester methyl group of both JH and its lower homologue JH-II. It does not contribute to the formation of the carbon skeleton. Farnesol, farnesyl pyrophosphate, and propionate are not utilized. Mevalonate is extensively incorporated into trans,trans-farnesol, but not into the sesquiterpene-like hormone. This result indicates that JH is not synthesized via mevalonate in the adult moths. Label of 2-14C-acetate was recovered in both JH and farnesol; the incorporation rate, however, was very small. The label of JH was located in the carbon skeleton.