In a Blazka swim tunnel swim trials with individual carp (Cyprinus carpio , N = 6, approximately 176 g) and rainbow trout (Oncorhynchus mykiss , N = 6 approximately 123 g) were performed until exhaustion at six body lengths per second (BL/s). Control carp (N = 6) and control trout (N = 6) were sampled after a moderate swim exercise at 1.5 BL/s. Significant differences were demonstrated in the exhausted carp group in comparison with the control group for plasma sodium (108.8%), lactic acid in red muscle (RM) (375.5%), lactic acid in white muscle (WM) (484.5%), triglycerids (133.9%), total protein (126.7%) and phospholipids (116.8%). In trout only, potassium was significantly elevated in the exhausted group (129.2%). T3- and T4-plasma values, as well as the T3/T4 ratio, were unaffected by the exercise protocol in both fish species. Despite the high lactic acid values in muscle tissue (RM: range 5-7 mM, WM: range 4-9 mM) in the exhausted groups of both fish species, the lactate in blood plasma in both fish species was not elevated (range 1.5-1.6 mM). This indicates that lactate is not released from the muscle compartment towards the blood and led to the concept of a 'non-release' lactic acid mechanism in cyprinid and a salmonid fish species after strenuous exercise.