We previously reported a higher blood viscosity at corrected hematocrit (45%) (explained by a higher value of erythrocyte rigidity) in football players with low serum zinc (Zn) and thus presumably Zn deficiency; subjects with low serum zinc had also an impairment in performance. This interventional study was undertaken in order to assess the effects of zinc supplementation (compared to placebo) on blood rheology and performance either at rest or during exercise. Ten male healthy volunteers (age: 26+/-1.3 yr; weight 67.9+/-2.24 kg; height 177+/-3 cm) received at random order either zinc (20 mg/day) and placebo, according to a double blind cross-over procedure, during seven days. In each case on the eighth day they performed a 25 min submaximal exercise-test. At rest blood viscosity at corrected hematocrit 45% (gamma = 1000 s(-1)) was lower after Zn (3.56+/-0.14 vs. 4.13+/-0.16 mPa.s, p = 0.009), explained by a lower RBC rigidity index 'k' according to Quemada's equation (1.65+/-0.07 vs. 1.84+/-0.08, p = 0.03). Hematocrit and plasma viscosity were unchanged, but RBC aggregation was decreased (laser retrodiffusion-derived aggregation time 'Ta' 3.52+/-0.51 vs. 2.75+/-0.59, p = 0.02). The increase in blood viscosity during exercise is lower after Zn than placebo. Blood viscosity at corrected hematocrit 45% remains unchanged during exercise after Zn, yet it increases after placebo. RBC rigidity index 'k' remains lower during exercise after Zn. The rating of perceived exertion (Borg's scale) at the 20th minute of exercise is lower after zinc (5.6+/-0.4 vs. 6.6+/-0.4, p = 0.008). This study confirms that Zn improves erythrocyte deformability, decreases the exercise-induced acute increase in blood viscosity, and improves exercise tolerance. Since Zn deficiencies are not unfrequent in sportsmen, these findings may be potentially relevant to sports nutrition.