Our purpose was to determine the plasma-volume (PV) change induced by a physical stress independent of the metabolic events that may interfere with physiological fluid shifts in to and out of the intravascular space. Our methods included using 178 exercise tests of varying duration and intensity for determination of PV change during exercise. Plasma Fourier transform infrared (FT-IR) spectra were used to compare hematocrit change to the total spectral area (4000-500 cm(-1)) and the protein and albumin concentration changes induced by exercise. Our results showed that exercise induced a raise in protein (+10.4 +/- 3.1%) and albumin (+9.8 +/- 3.3%) concentrations that significantly correlated with PV change (14.1 +/- 5.2% of plasma volume; P = 0.05 with protein and albumin concentration changes). However, evolution of the total spectral area obtained from rest-plasma (524 +/- 21 a.u.) and exercise-plasma (611 +/- 26 a.u.; +14.1 +/- 4.8%) FT-IR spectra showed a higher correlation level with PV change (r = 0.98; P = 0.005; S(x/y) = 1.26 a.u.). It is our conclusion that although exercise-induced changes in protein and albumin concentrations were found to correlate with PV change, the use of the total spectral area of the plasma FT-IR spectra allowed a more precise measurement of PV change.