We have investigated the activity and abundance of a number of respiratory chain components in ripening and cold-treated tomato fruits (Lycopersicon esculentum L. Mill cvv. Moneymaker and Sweetie). Expression of the alternative oxidase (AOX) protein increased dramatically in both situations. Levels of the plant uncoupling protein (UCP) initially fell, but increased substantially in the later stages of ripening. In contrast, ATP synthase subunits and the COXII subunit of cytochrome oxidase decreased during ripening and increased slightly in response to cold stress. Other proteins involved in electron transport, tricarboxylic acid cycle function, chaperonin function, and membrane transport were also studied. These showed varying degrees of enhanced and depressed expression patterns. There were modest changes in whole fruit respiratory activities, and electron transport capacity of isolated mitochondria in response to these stimuli. However, respiratory control by ADP in the isolated mitochondria decreased as AOX capacity and abundance increased, indicating that although total respiration rates changed little, flux between the coupled and uncoupled pathways altered. The changes observed in AOX and UCP accumulation in tomato fruit that were vine-ripened were significantly different from post-harvest ripening patterns previously reported. The altered protein profiles are discussed in the context of on- and off-vine ripening and the potentially different roles of uncoupled respiration in each situation.