The aim of the present study was ascertain the effects posture and exercise modality on the main determinants of blood rheology. Thirteen subjects performed two exercise trials, in random order, at approximately 70% VO(2) max for 45-min. One trial was performed on a motorized treadmill at an intensity corresponding to 70% VO(2) max, while the other was performed on a stationary pike at an intensity corresponding to 70% VO(2) max. In the cycling trial subjects stood for 30-min, followed by sitting for 30-min then cycled for 30-min at 70% VO(2) max. In the treadmill trial, subjects sat for 30-min followed by standing for 30-min then ran on the treadmill for 30-min at 70% VO(2) max. Variations of body postures prior to exercise were associated with opposite changes in plasma volume, plasma viscosity and plasma fibrinogen. When post exercise raw data were not adjusted for plasma volume changes, a significant increase (p < 0.05) in plasma viscosity and plasma fibrinogen was found with no difference between the cycling and running trials. However, the increase in plasma viscosity and fibrinogen were no longer apparent when the raw data post exercises were adjusted for plasma volume changes. Changing body posture from standing to sitting and vice versa were associated with opposite changes in plasma volume and mirrored the changes in plasma viscosity and fibrinogen. In addition, ergometer-specific vigorous exercises at the same relative intensity, irrespective of its modality, transiently increased plasma viscosity and plasma fibrinogen mainly due to exercise-associated haemoconcentration.