Freshly harvested early- and mid-season Bartlett pears (Pyrus communis) were treated with ethylene (air plus 10 Pa C2H4) or air at 5, 10, and 20 degrees C for 24 and 48 h (experiment 1) and at 5 and 10 degrees C for 48, 72, and 96 h and at 20 OC for 24 h (experiment 2). Following C2H4 or air treatment at different temperatures and durations, pears were transferred to 20 degrees C in air for ripening. Bartlett pears were evaluated for firmness, color, respiration, C2H4 production, and activities of 1-aminocyclopropane-1-carboxylic acid synthase (ACC-S) and 1-aminocyclopropane-1-carboxylic acid oxidase (ACC-O). Ethylene action was temperature dependent. The duration of C2H4 conditioning needed to fully induce ripening was longer at lower temperatures: 72 h at 5 degrees C, 48 h at 10 degrees C, and 24 h at 20 degrees C. Cold storage in air for as little as 3-4 days at 5 or 10 OC appeared to hasten subsequent ripening, but to a lesser extent than pears kept for 2 weeks at -1 degrees C in air. Despite a significant increase in ACC-S activity in pears treated with C2H4 at 5 OC, there was not a simultaneous increase in ACC-O activity, resulting in low C2H4 production that was insufficient to generate the threshold endogenous levels of C2H4 required for ripening. Contrary to previous findings with pears, these data indicate that ACC-O could be a rate-limiting step in C2H4 biosynthesis.