The retarding effect of CO₂ on senescence processes in leaves was studied by applying 10% CO₂ to chervil bunches in a flow-through system. Since the senescence retarding effect of CO₂ could be attributed to its action as a competitive ethylene inhibitor, the effect of CO₂ on chervil leaf senescence in the presence (2000 μl/l) or absence of another ethylene action inhibitor, 2,5-norbornadiene (NBD), was studied. Results show that senescence processes (chlorophyll loss and lipid oxidation) were more effectively retarded in CO₂- or CO₂ + NBD-treated leaves, as compared with NBD-treated leaves. These results indicate that the senescence-retarding activity of CO₂ cannot be solely attributed to its action as an anti-ethylene agent. Since high levels of CO₂ would presumably affect internal pH, with the active species of CO₂ hydration being HCO^3- or H+, a possible mechanism for the ethylene-independent action of CO₂ in delaying senescence through shifting of cellular pH, is suggested. CO₂ treatment resulted in increase of the cell sap pH of chervil leaves by one pH unit during senescence. This increased pH was mainly due to retention of the original levels of polyamines in CO₂- or CO₂ + NBD-treated chervil bunches in the course of senescence. During this period, the level of polyamines decreased by 50% in control or NBD-treated leaves. It seems therefore that in addition to its anti-ethylene action, the senescence-retarding effect of CO₂ operates through maintenance of the original levels of polyamines, which acted as senescence retardants in chervil leaves. This retention of polyamines, which are known as nitrogenous bases, may exhibit a possible mechanism for regulating the cytoplasmic pH as a response to the transitional cellular acidification by high CO₂.