The activities of the cytochrome and alternative respiratory pathways were measured during the growth cycle in Arabidopsis thaliana using a newly developed Isotope Ratio Mass Spectrometer (IRMS) dual-inlet system that allows very precise measurements of oxygen-isotope fractionation under low oxygen consumption rates. Under optimum growth conditions, the relative growth rate was highly dependent on the activity of the cytochrome pathway. The activity of the alternative pathway was almost constant irrespective of the growth rate and appeared mostly involved in the maintenance respiration component, although the alternative pathway also played a role under optimum growth conditions. This is the first time that the contribution of the two respiratory pathways to the two main respiratory components (growth and maintenance) has been analyzed with the use of the oxygen-isotope fractionation technique. The respiration efficiency of the specific costs for growth and maintenance (adenosine triphosphate/O2 ratio) was determined by a modified regression model. The ability to measure oxygen-isotope fractionation during respiration in A. thaliana opens the door to a wider set of studies that are discussed.