To clarify the ripening phenomena with special reference to the effect of endogenous C2H4 on respiration in Golden Delicious, Starking Delicious and Fuji apples, fruit samples were taken at 7-14 day intervals with a total of 3 to 6 harvests during the period of maturation and ripening. The rates of respiration and C2H4 evolution, and the internal gas concentrations were measured for 7 to 18 days beginning 1 day after harvest, and the specific gravity and watercore rating were also measured at harvest or 1 day after harvest.1. With the exception of late harvested Fuji apples, an increase in internal CO2 concentration corresponded with a rapid increase in internal C2H4 concentration, and with a little delay a rise in respiration occurred. In Starking Delicious apples, a decline in internal CO2 concentration started during the rise in respiration, and there occurred decreases in respiration and internal C2H4 concentration at about the same time, and subsequently a decline in C2H4 evolution began. On the other hand, in Fuji apples a decline in internal CO2 concentration occurred at about the same time as a decline in respiration, and then an internal concentration of C2H4 began to decline, followed later by a decline in C2H4 evolution.2. An increase in specific gravity associated with watercore development accompanied a marked decline in internal O2 concentration and an increase in internal CO2 concentration. In Fuji apples, as harvest was delayed, there was an increase in specific gravity with a corresponding increase in the internal concentration of C2H4 at 1 day after harvest. Late harvested Fuji apples, which had a high specific gravity and severe watercore, exhibited little or no rise in respiration even if there occurred a rapid increase in internal C2H4 concentration, which was not observed in the case of a low internal concentration of O2.3. It is considered from these data that C2H4 elevates the rate of respiration and induces the climacteric rise in respiration, but the increase in specific gravity or the development of watercore causes changes in gas permeability and solubility of apple tissues, and thereby exerts great influence on the rates of respiration and C2H4 evolution, and the internal gas concentrations as well as the effect of C2H4 on respiration.