Effects of exercise on erythrocyte aggregation were investigated in 19 elite athletes. High shear rate viscometry (1000 s− 1) evidenced an increase in blood viscosity explained by an increase in hematocrit (+ 8% p< 0.01) and plasma viscosity (+ 7% p< 0.01). Erythrocyte rigidity index and erythrocyte aggregability measured using the Myrenne erythroaggregometer did not change. However, using the laser backscattering technique (SEFAM erythroaggregometer), we observed significant changes in aggregability and desaggregability after 25 min of exercise. The initial aggregation time (TA) decreased by 33%(p< 0.01), while the final aggregation time decreased by 13.6%(p< 0.01). TA was correlated with areobic working capacity (r= 0.73; p= 0.005), which was negatively correlated with blood viscosity at rest (r=− 0.57; p= 0.043). A significant relationship was observed between TA and the initial fibrinogen levels (r= 0.71; p< 0.01). The plasma volume contraction during exercise was found to be statistically explained by the water loss proportional to the total work load. Thus, laser backscattering demonstrates an increase in aggregability and a decrease in disaggregability of red cells during exercise, proportional to baseline fibrinogen values.