The potential usefulness of substrates for glycolysis and nucleic acid synthesis as tumor imaging agents was compared to that of 67Ga-citrate. In separate experiments, 3H-thymidine, 3H-uridine, 14C-2-deoxyglucose, and 67Ga-citrate were injected intraveneously into BALB/c mice with solid subcutaneous EMT-6 sarcomas. For the 3H- and 14C-labeled substrates, absolute uptakes in tumor and tumor-to-blood ratios were as high 1 hour after injection as the comparable maximum values achieved for 67Ga-citrate after 48 hours. These studies suggest that positron-labeled analogues of thymidine, uridine, and 2-deoxyglucose should be useful radiopharmaceuticals for tumor imaging by positron-emission tomography.