1. The absorption and glucuronylation of [14C]diethylstilbestrol (DES) were studied in situ in segments of rat small intestine. 2. DES glucuronide accounted for nearly 50% of the methanol-soluble radioactive material appearing in the intestinal venous blood. 3. There was no difference between DES glucuronide formation in intestinal segments near the pyloric sphincter and in segments located near the ileo-caecal junction. 4. The appearance of DES and its glucuronide in the venous blood of aborally-located segments was dose-dependent from 0.1 to 5 mM, but in orally-located segments, an increase in DES concn. from 2 to 5 mM produced no increase in DES or its glucuronide in the venous blood. In neither of these intestinal regions was the rate of appearance of DES or DES glucuronide dependent upon intestinal blood flow. 5. The results indicate that intestinal glucuronylation of certain compounds during absorption may be significant over a wide range of concn. in all regions of the intestine.