Ten subjects were studied in order to determine whether mild exercise after an overnight fast would influence the baseline breath H2 concentration or breath H2 response to the nonabsorbable disaccharide, lactulose. Breath H2 concentrations immediately after exercise were significantly lowered (p less than 0.0001) but rapidly returned to baseline values. Exercise did cause a significant change in the overall response after oral lactulose and must be considered when sequential breath H2 tests are undertaken.