The fecal tryptophanase activities were 0.267±0.10 for rats and 0.185±0.01 μmole/min/g wet feces for humans. The activities of indole pyruvate degradation to indole, indole pyruvate lyase, of these feces were 0.051±0.02 and 0.046±0.01 μmole/min/g wet feces, respectively. The optimal pH values of tryptophanase and indole pyruvate lyase were 5.5–7.5 and 5.5–6.5, respectively. When the intestinal flora orE. coli HGU-3 was cultured in GAM broth having six different pH values (5 to 10), the activities of tryptophanase and indole pyruvate lyase in the medium adjusted at pH 6 were dramatically induced by elevating the pH to 9. However, when intestinal microflora were inoculated in the medium containing lactulose, the productions of these enzymes were dramatically inhibited and the pH of the medium was lower than that of the control.