Indium-111-diethylenetriaminepentaacetic Acid-D-phenylalanine1-octreotide (111In-DTPA-octreotide) is a cyclic eight amine acid somatostatin analogue which is approved for gamma scintigraphy of neuroendocrine tumors. To address the factors that contribute to liver and kidney retention of this radiopharmaceutical, its metabolism was evaluated in normal and tumor-bearing rats. The soluble fractions from nontarget (liver and kidney) and target (tumor, pancreas, adrenals) organ homogenates were analyzed out to 21 h postinjection, and urine was analyzed out to 12 h postinjection. The blood was analyzed at shorter time intervals due to the rapid clearance of 111In-DTPA-octreotide. RadioTLC and HPLC were used to analyze organ homogenates, blood, and urine. By TLC, intact 111In-DTPA-octreotide was resolved from the soluble metabolites, and a similar apparent rate of metabolism was observed in the liver, kidney, tumor, and pancreas with ~30% intact 111In-DTPA-octreotide at 4 h postinjection. In the adrenals, metabolism occurred more slowly with ~60% intact 111In-DTPAoctreotide at 4 h postinjection. At 4 h postinjection, the activity excreted in the urine consisted of 85% intact 111In-DTPA-octreotide. HPLC provided resolution of the individual extractable metabolites. In an attempt to identify these metabolites, two DTPA-amino acid sequences were synthesized: DTPAD-Phe-Cys and DTPA-D-Phe. Under the conditions used for metabolite analysis, 111In-DTPA-D-PheCys-OH eluted at 14.6 min and 111In-DTPA-D-Phe-OH eluted at 7.0 min. Each of these standard sequences was combined with the soluble portion of the organ homogenate and was shown by HPLC to coelute with the metabolites. These data suggest that 111In-DTPA-octreotide was initially degraded to 111In-DTPA-D-Phe-Cys-OH and 111In-DTPA-D-Phe-OH. The 111In-DTPA-D-Phe-Cys-OH was further degraded to 111In-DTPA-D-Phe-OH, which appeared to be the final metabolite that was extracted from the organs. From these results, it can be concluded that at longer time points (>2 h postinjection) a significant amount of 111In was retained in nontarget organs as 111In-DTPA-D-Phe-OH and 111In-DTPAD-Phe-Cys-OH and not as intact 111In-DTPA-octreotide.