A decrease in the elevation of the linear relation of heart rate on oxygen consumption (HR/VO2) throughout submaximal exercise testing has previously been used in individuals as an index of improvement in cardiorespiratory fitness after a moderate intensity training programme. In the present investigation, changes in HR/VO2 elevation and its variability were quantified and compared with those of other similar indices. These comprised changes in the level of oxygen consumption or heart rate attained at respiratory exchange ratio (R) of unity (VO2/R=1 or HR/R=1. In a first trial in six healthy volunteers, the decrease in HR/VO2 elevation caused by 16 weeks of training significantly exceeded measurement variability, and was associated with increases and decreases respectively in VO2/R=1 and HR/R=1. In a second trial in eight healthy volunteers, the three indices were obtained at four stages during the course of the 16 weeks of training, to grade the improvement in fitness. There were progressive changes in the three indices, the magnitude of which was greatest in the case of the HR/VO2 index. The findings make it possible quantitatively to obtain a graded measure of improvement in fitness during exercise training as indicated by the magnitude of decreases in HR/VO2 elevation.