Carbon dioxide-enriched atmospheres can be effective in the retardation of ripening and in the reduction of decay of horticultural commodities. However, concentrations in excess of the tolerance level may cause physiological damage. The goal of our research is to elucidate the specific regulatory mechanisms of CO2 actions. Cytochrome oxidase (CytOx) in vitro activity in preclimacteric avocado fruit stored in air or 40% CO2 + 12.6% O2 was evaluated at 20C. Activities were determined during treatment and also after a transfer to air. Fruit treated with 40% CO2 + 12.6% O2 had elevated CytOx in vitro activity when compared to air-stored fruit. Immunoblot analysis was performed to determine if the increase in CytOx activity could be due to an increase in enzyme concentration. The decline in respiration rate of CO,-treated fruit was most likely due to the decrease in intracellular pH and its effect on the activities of important respiratory enzymes, including CytOx. The regulatory mechanisms of other mitochondrial respiratory enzymes in `Hass' avocados exposed to elevated CO2 atmospheres are also under investigation.