The effects of resistance exercise upon leucine oxidation and whole body protein synthesis were studied using stable isotope methodology. L-[1-13C]leucine was used as a tracer to calculate leucine oxidation and whole body protein synthesis in six healthy, fed, male athletes in response to a 1 h bout of circuit-set resistance exercise. The measurements were performed prior to, during, and for 2 h after exercise, and corrections were made for background 13CO2/12CO2 breath enrichment and bicarbonate retention factor changes. Results demonstrated significant (P less than 0.01) increases in the background 13CO2/12CO2 breath enrichment at 1 and 2h after exercise and in the bicarbonate retention factor (P less than 0.01) during exercise. At 15 min after exercise, the bicarbonate retention factor was significantly (P less than 0.05) lower than at rest. There were no significant effects of exercise on leucine oxidation or flux, whole body protein synthesis, or the rate of appearance of endogenous leucine. We concluded that circuit-set resistance exercise did not affect the measured variables of leucine metabolism. In addition, large errors in calculating leucine oxidation and whole body protein synthesis during resistance exercise can occur if background 13CO2/12CO2 breath enrichment and bicarbonate retention factor changes are not accounted for.