Since CO2is liberated in the conversion of ACC to ethylene, the evidence that ethylene production by plant tissues is actually promoted by CO2calls for an explanation. Accordingly, the formation of ethylene by oat (Avena sativaL. cv. Victory) leaves and by apple (Golden Delicious) and pear (Pyrus communisL. cv. Anjou) tissue in very low levels of CO2has been studied. The gas chromatograph was modified to measure CO2and ethylene at the same time, by reducing both to methane. (Response of the gas chromatograph to CO2concentrations is linear.) The work establishes a clear difference between the endogenous production of ethylene and its production from applied ACC, a difference which holds about equally for leaves and for fruit tissue. The difference is in the CO2requirement, namely, lowering the CO2level by 99% or more decreases the production of ethylene from applied ACC by 50–60%, but it does not decrease, or even slightly increases, its production from endogenous precursors. Thus, while the need for CO2has not been explained, it has at least been delimited.The responses to abscisic acid (ABA) also differ, but in the reverse direction, the endogenous production of ethylene being decreased up to 70% by ABA. while the liberation from ACC is promoted about 20%. ABA also promoted the respiratory CO2production by 30% or, in presence of 1‐aminocyclopropane‐1‐carboxylic acid (ACC), by over 50%. Inhibition of ethylene production by cobalt or EDTA shows no such differentiation, while inhibition by Na catechol‐4,6‐disulfonate (Tiron) shows a small difference. It is concluded either that endogenous ethylene is formed by an enzyme system different from that reacting with ACC, or (more likely) that when ethylene arises from endogenous precursors the reaction does not proceed by way of free ACC, but by some activated