The thymidine analog 2′-fluoro-5-methyl-1-β- d-arabinofuranosyluracil (FMAU) is incorporated into DNA and is resistant to catabolism. We performed pharmacokinetic measurements with 14CFMAU and PET studies with 11CFMAU using rats bearing several different syngeneic tumors. Among normal tissues, FMAU uptake reflected relative cell turnover rates. Among tumors, the highest uptake occurred in a rapidly growing colon carcinoma, but was similarly low in both rapidly and slowly growing prostate tumors. FMAU was not catabolized and was rapidly incorporated into DNA by small intestine and colon tumors. Results indicate that FMAU may be useful for imaging tissue DNA synthesis, although tumor uptake was modest and not well correlated with growth rate among the models examined.