To determine the site of 3-methylindole (3MI) and indole absorption in cattle after ruminal administration of L-tryptophan (TRP), 4 Holstein steers were given 0.4 g of TRP/kg of body weight directly into the rumen through ruminal cannulas. Chromium EDTA and ruthenium phenanthroline were added to feedings of orchard grass hay twice a day for measurement of fluid and particulate flow to the duodenum, respectively. Passage of 3MI and indole (products of ruminal fermentation of TRP) to the duodenum was determined by the products of digesta flow rate and concentration in duodenal contents. Ruminal fluid, duodenal contents, and jugular blood were sampled at postdosing hours (PDH) 0, 6, 12, 18, 24, 36, and 72 for analysis of 3MI, indole, and digesta flow markers. Ruminal, duodenal, and jugular plasma concentrations of 3MI and indole peaked at PDH 12 to 24 at 152.4 and 25.9; 15.5 and 1.0; and 8.7 and 2.2 mg/L, respectively. Most 3MI and indole reaching the duodenum were associated with the particulate phase of the digesta. On a molar basis, total passage of 3MI to the duodenum during 72 hours amounted to 1.0% of the TRP dose for 3MI and 0.1% of the TRP dose for indole. Absorption of 3MI and indole in these steers was almost entirely proximal to the duodenum.