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To determine if the time courses of hemorheologic parameters are different between carriers of sickle cell trait (SCT) and subjects with normal hemoglobin in response to exercise.Observational and comparative study.Testing was conducted in a laboratory of exercise physiology.Nine carriers of sickle cell trait (SCT group) and 7 subjects with normal hemoglobin (CONT group) performed an exercise protocol of the repetition of 3 successive maximal ramp exercise tests.Blood was sampled at rest (TR), at the end of each of the 3 tests (T1, T2, T3), and during the immediate (T2h) and late (T24h, T48h) recovery periods.Blood and plasma viscosity (ηb and ηp, respectively), hematocrit (Hct), and red blood cell (RBC) rigidity (Tk and k indexes) were determined.In both groups, ηb significantly increased in response to exercise but the SCT group had significantly higher ηb at T3 and T2h. ηb then returned to baseline value at T2h in the CONT group and at T24h in the SCT group. Tk and k were not changed by exercise but significantly increased above baseline value in both groups at T24h and T48h. The increase in Tk and k during late recovery was higher in the SCT group than in the CONT group, indicating that SCT carriers had significantly higher RBC rigidity than the CONT group at that time.The hemorheologic changes induced by exercise in the SCT carriers could trigger microcirculatory disorders during the recovery.