Brain and tumor uptake of L-[methyl-3H]methionine (3H-Met), L-[1-14C]leucine (14C-Leu) and L-2-[18F]fluorotyrosine (18F-Tyr) and their incorporation into the acid-precipitable fraction (APF) were investigated in mice bearing FM3A-P0 and the higher metastatic FM3A-P15A mammary carcinomas in order to compare the potential of positron emitting analogs for measuring protein synthesis rates (PSR) in brain and tumors by positron emission tomography (PET). The total uptake of the 3H-Met by the brain was higher than those of two other amino acids. On the other hand, the incorporation rates into the APF in the brain and tumors were significantly faster with 14C-Leu compared to the others. Most of the 14C in the APF was identified in labeled proteins; however, considerable amounts of 3H were recovered in the lipid and RNA fractions, and 18F was also detected in the lipids. Among the positron emitting analogs of three amino acids, 11C-labeled Leu could be the most suitable for measuring PSR by PET. For all the amino acids, the total uptake and the APF both expressed as the % injected dose seemed slightly higher in the FM3A-P15A than in the FM3A-P0, although the differences did not reach statistical significance.