The correlation between relative haemoglobin mass (Hb mass, g x kg[-1]) and relative maximal oxygen consumption (VO2max, ml x kg(-1) x min[-1]) in 62 trained athletes (33 male runners, 12 male rowers and 17 female rowers) with national and/ or international competitive experience was examined. The correlation between Hb mass and VO2max was highest for the female rowers (n=17, r=0.92, p<0.0001), lower for the male rowers (n = 12, r=0.79, p < 0.005) and lowest for the male runners (n=33, r=0.48, p = 0.005). These results suggest that, within an athletic sample, Hb mass may be used to estimate potential aerobic power. In a second series of experiments, Hb mass was measured before and after three different training programs in sub-sets of the subjects used in the earlier study. Hb mass did not change following 12 weeks of intense rowing training, 4 weeks of heat training (32 degrees C), or 4 weeks of medium-altitude training (1740 m). The corresponding increases in VO2max were 7.8%, no change and 2.1 %, respectively. These results suggest that heat or altitude training does not increase Hb mass in trained athletes. Previous studies that demonstrate increases in total red cell volume following altitude acclimatization used subjects with only modest aerobic power, whereas the present study used trained subjects. It is concluded that trained athletes with erythrocythemic hypervolemia have limited capability to increase further either total red cell volume or Hb mass.