In previous studies, water stress has induced variable and sometimes contradictory changes in respiration. We used isothermal calorimetry to measure the response of foliar respiration to water deficit in nine eucalypt genotypes. Specific growth rates (R-SG) of shoots and leaves of variable age were measured independently, and the data were applied to both the growth-maintenance and enthalpy balance models. We calculated the oxidation state of respiratory substrate and the enthalpy change for the conversion of substrate carbon to biomass (Delta H-B). Moderate water stress reduced the R-SG of shoots by 38% (P <0.01) and carbon conversion efficiency by 15% (P <0.05). The relationship between carbon conversion efficiency and R-SG was not affected by water deficit for shoots, but was significantly altered for leaves. Water deficit increased maintenance respiration by about 23% (P <0.001). The growth coefficient of respiration was not significantly altered. However, changes in oxidation states of substrate and biomass suggest that the energy requirements of biosynthesis were increased under water stress. Our results confirm that carbohydrates are the major respiratory substrates in growing tissues, though mature leaves utilized a substantial component of more reduced substrate. Mature leaves had variable oxidation states for respiration substrate, which indicates a variable relationship between CO2 evolution and ATP production. Measured Delta H-B in shoots and leaves were too small for reliable estimation of R-SG by the enthalpy balance model. We also found significant effects of water stress on the oxidation state of substrate and Delta H-B.