This study examined exercise-induced hypoxaemia (EIH) and plasma volume contraction as modulators of serum erythropoietin (Epo) production. Five athletes cycled for 3 min at supra-maximal power outputs, at each of two different elevations (1,000 m and 2,100 m). Five subjects were exposed to normobaric hypoxia (F(I)O(2)=0.159), seven subjects underwent plasmapheresis to reduce plasma volume and eight subjects were time controls for Epo levels. Oxyhaemoglobin saturation was significantly reduced during exercise and during normobaric hypoxia. The time period of haemoglobin oxygen saturation <91% was 24+/-29 s (mean+/-S.D., n=5) for exercise at 1000 m, 136+/-77 s (mean+/-S.D., n=5) for exercise at 2100 m and 178+/-255 s (mean+/-S.D., n=5) with resting hypoxic exposure. However, significantly increased serum Epo levels were observed only following exercise (24+/-3%; mean+/-S.D., n=5 at 1,000 m and 36+/-5%; mean+/-S.D., n=5 at 2,100 m). Volume contraction also resulted in increased serum Epo (35+/-6%; mean+/-S.D., n=7) in spite of a significant rise in haematocrit of 2.2%. Despite similar degrees of arterial desaturation, only the hypoxaemia induced by exercise was associated with an increase in serum Epo. This finding indicates that other factors, in addition to hypoxaemia, are important in modulating the production of Epo in response to exercise. Volume depletion in the absence of exercise resulted in increases in Epo levels that were comparable with those observed in response to exercise. The paradoxical responses of the increased haematocrit and the increase in Epo in subjects undergoing plasmapheresis suggests that plasma volume may also modulate the production of Epo.