To develop ligands for imaging breast tumors, [F-18]fluoro analogue of tamoxifen and [F-18]fluoroalanine were radiosynthesized. In vivo biodistribution studies were performed in mammary tumor-bearing rats. In studies on the biodistribution of an [F-18]fluoro analogue of tamoxifen, tumor uptake decreased when rats were pretreated with diethylstilbestrol (DES), suggesting that tracer uptake in tumors was receptor-mediated. An estrogen receptor assay indicated that tumors have a receptor density of 7.5 fmol/mg protein. Studies of the distribution of [F-18]fluoroalanine in tissue showed that the tumor-to-tissue ratio increases as a function of time. Positron emission tomography (PET) images of tumor-bearing rats demonstrated that tumors can be visualized 1 h after rats are injected with an [F-18]fluoro analogue of tamoxifen. PET imaging of pigs after injection of 10 mCi of [F-18]fluoro analogue of tamoxifen showed uterine uptake that could be blocked by DES (50 mg). The findings suggest that both radiotracers are useful for imaging breast tumors.