5-thio-D-glucose (5-TDG) exerts profound effects on rapidly metabolizing tissues. We have used liquid scintillation counting to study the tissue distribution and pharmacokinetics of S-35-labeled 5-TDG in hamster models of pancreatic tumors. In the normal hamster, initial uptake of S-35 activity into kidney, liver, and blood was high, but rapidly decreased with time. The pancreatic uptake (% dose/g) never exceeded 0.75%. This level occurred only at the earliest times after administration. Uptake in all three tumor models exceeded that in pancreatic tissue within 15 min of injection. The highest tumor-to-pancreas ratio was seen in the duct-tumor model, which also exhibited the most favorable tumor-to-tissue ratio when compared with kidney, liver and muscle. Favorable ratios were most pronounced at 6 and 24 hr after injection. These studies provide impetus for the use of 5-TDG as a model compound for the synthesis of potentially useful agents for clinical detection of pancreatic tumors.