Understanding age-related changes in respiratory parameters is required if measurements of juvenile metabolic properties are to be used for predicting long-term growth rates of trees. This study examines the influence of tree age on respiration in 13 Eucalyptus species, in different genotypes of the same species, and in rooted cuttings (ramets) of a single genotype. All of the respiratory parameters measured - metabolic heat rates, CO2 production rates, and temperature coefficient of heat rate showed systematic changes with tree age. Substrate carbon conversion efficiency and specific growth rates calculated from the respiratory data generally decreased with tree age.