Partially ripened avocado [Persea americana (Mill.) cv. Hass] fruit harvested in either June or Aug. 1994 were kept at 10 °C in air (21% O2), 20% CO2 (17% O2, balance N2), or 40% CO2 (13% O2, balance N2) for 7 to 12 days and then were transferred to air at 10 °C for 2 to 3 days. Mitochondrial respiration was stimulated in response to elevated CO2 treatments at 10 °C. A shift to alternative pathway (Alt) respiration occurred on day 4 in experiments using avocados from both harvest dates, with a return to initial levels in only the 20% CO2-treated fruit (June-harvested fruit after return to air). Elevated CO2 at 20 °C decreased the in vitro O2 consumption of isolated mitochondria compared to mitochondria kept in air. The Alt pathway contributed less to the total O2 uptake of CO2-treated mitochondria compared to mitochondria kept in air. The respiratory control ratios of the CO2-treated fruit and mitochondria were higher and lower, respectively, than the air controls. Induction of 33 to 37 kD proteins (corresponding to the size of the alternative oxidase proteins) occurred in avocados after 4 days in 40% CO2. These results indicate that elevated CO2 has various effects depending on concentration, duration and temperature of exposure, and mitochondrial function of avocado fruit, such as increased and altered respiratory oxidation and up-regulation of alternative oxidase proteins.