ABA and ethylene treatments were applied to preclimacteric `Granny Smith' apples [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] harvested at three different maturity stages. Ethylene production rates, ethylene-forming capacity (EFC), free and conjugated ACC contents, presence of ACC oxidase (ACO) and ripening-related ACC synthase (ACS) proteins, and endogenous ABA levels were monitored at harvest and during 3 weeks thereafter. ABA treatment resulted in a specific accumulation of ACO protein and of ACS-related polypeptides in fruit collected ≈2 months before commercial harvest, whereas the same tissues showed no response to exogenous ethylene. In contrast, fruit harvested 1 month later proved more sensitive to ethylene but not to ABA, in accordance with evolution of endogenous ABA levels, which were highest at this maturity stage and were enhanced in response to exogenous ethylene. A possible role for ABA as an inductor of the competency to ripen is discussed. Chemical names used: abscisic acid (ABA); 1-aminocyclopropane-1-carboxylic acid (ACC).