`Castlemart' tomato (Lycopersicon esculentum Mill.) pericarp discs were used to study the physiological effects of acetaldehyde and ethanol on fruit ripening. Short-term exposure of discs from mature-green fruit to acetaldehyde vapors on a fresh mass basis (≤500 μg·g-1) or ethanol vapors (≤3 mg·g-1) promoted ripening, while higher concentrations inhibited ripening. Discs from mature-green fruit absorbed greater amounts of ethanol and produced significantly higher concentrations of acetaldehyde than discs from breaker fruit. Ripening was promoted by ethanol when the discs were unable to retain or produce a certain level of acetaldehyde. Inhibition of ripening by 4 hours of exposure to ethanol (6 mg·g-1) was almost completely abolished by hypobaric treatments (18 kPa for 24 hours). However, acetaldehyde-induced ripening inhibition (2 days exposure to 180 μg·g-1) was only slightly reduced by vacuum. Concentrations of acetaldehyde and ethanol that inhibited ripening reduced C2H4 production, whereas lower concentrations of acetaldehyde and ethanol that promoted ripening increased C2H4 production. Application of 4-methylpyrazole, an alcohol dehydrogenase inhibitor, enhanced acetaldehyde-induced ripening inhibition and reduced ethanol-induced ripening inhibition or promotion at all concentrations of acetaldehyde and ethanol tested. The inhibition or promotion of ripening of excised tomato pericarp discs by ethanol and acetaldehyde depended on initial fruit maturity, applied volatile concentration, and duration of exposure.