Some physiological changes in mechanically injured apple, pear and peach fruits were studied. The bruised fleshes of the fruits showed remarkable wound responses which consist of an accelerated CO_2 output, an increased intensity of ethanol production and a decreased acid contents. The acceleration in CO_2 output was seen most remarkably within 24 hours after bruising and lasted for at least 2 days. When 'KOKKO' apple fruit was dropped onto a hard board from the height of 1 m, CO_2 output began to be accelerated 1.5-2.0 hours after bruising Under such circumstances as the surfaces of bruised regions of 'KOKKO' apple and 'OKUBO' peach fruits were covered with 6x6 <cm>^2 adhesive tapes and the exchanges of gas through them were consequently cut off, the acceleration in CO_2 output couldn't be observed imrnediately after bruising at least. The decreased organic acids inbruised fleshes of fruits were malic acid and citric acid in 'OKUSANKITI' pear and malic acid in 'KOKKO' apple In 'KOGYOKU' apple fruit, acids contents in the bruised fleshes of the fruits were decreased rapidly at 25℃ and gradually at 5℃, which showed the dependence of the acids contents in bruised flesh of the fruit to temperature. Ethanol production in bruised flesh of the fruit was accelerated in an atmosphere of nitrogen. There was no correlation between ethanol accurnulation and temperature. When it was assumed that the additional C0_2 produced as a result of bruising injury arises from oxidative decomposition of organic acids and alcoholic fermentation, the value of additional CO_2 calculated from the decreased acid contents and the accumulated ethanol contents in bruised flesh of the fruit approximately agreed with the experimental value. In an atmosphere of nitrogen, however, CO_2 output wasn't always accelerated in spite of constant decrease of acid contents and ethanol accumulation in bruised flesh of the fruit, which didn't show the correlation of accelerated CO_2 output with decreased acids or with accurnulated ethanol.