In the present study, hemoglobin levels were not significantly different between the high and low NO3-N treatments.There was no evidence of methemoglobinemia, as hemoglobin concentrations for both treatments were actually elevated and abovethe range reported as normal in rainbow trout, 8.9–15.9 mmol/L(Wedemeyer, 1996). Davison (1997) reported that exercise train-ing increases hematocrit and thus hemoglobin concentration in theblood; thus the elevated hemoglobin concentrations measured forboth treatments were likely related to continuous exercise and therapid swimming speeds observed