Recent studies employing gas chromatography show that an amount of ethylene large enough to stimulate ripening is always present within a fruit before the respiratory climacteric begins. This fact and data from experiments in which fruits were exposed to a partial vacuum or varying concentrations of O(2), CO(2), and ethylene oxide reinforces the view that ethylene is a ripening hormone. The respiratory climacteric begins soon after the fruit is harvested because the tissue no longer receives from the shoot system a substance which inhibits ripening; this substance may act by lowering the sensitivity of the fruit to ethylene. The threshold for ethylene action is also influenced by the composition of the atmosphere, for O(2) is a substrate in the reaction activated by ethylene and CO(2) inhibits the action of ethylene by competing with the olefin for the receptor site. Experiments indicate that ethylene is derived from acetate or acids of the Krebs cycle and acts by binding to a metal receptor site in the tissue.