Recently extra-cellular Lac was shown to have protective effects on muscular performance in vitro. Whether this could be shown in vivo in humans was aim of the study. 8 subjects performed handgrip interval exercise of maximum intensity under 2 conditions (with and without acidosis). Acidosis was induced by cycle ergometry (2 min with max power reached in an incremental test). Hand grip exercise started 5 min after terminating cycle exercise. Hand grip power was measured with an inductive device. Blood chemical parameters were measured in cubital venous and arterialized blood. When forearm exercise started, [Lac] in arterialized blood was 10.9 +/­1.2 mmol/l (aci) compared to 1.3 +/­0.3 mmol/l (con). Venous pH was lowest during the 2nd exercise bout (aci: 6.99 +/­0.03; con: 7.15 +/­0.02). As PCO2 was similar at that time the pH difference is due to lac. Venous [K+] was similar, as well. In spite of the large difference in pH neither a beneficial nor a detrimental effect on performance could be seen. The difference in pH between both conditions decreased slightly with the number of intervals but remained significant (p<0.001). Performance again was not influenced. Thus under our in vivo conditions, we could show neither a protective nor an fatiguing effect of extra-cellular lactic acid on muscles working with maximum intensity.