We investigated the potential of O-[(11)C]methyl-L-tyrosine and O-[(18) F]fluoromethyl-L-tyrosine as positron-emitting tracers for tumor imaging. The two tracers had similar distribution patterns in rats bearing AH109A hepatoma, with pancreas and, on a lesser extent, AH109A showing the highest uptake. Uptake of both tracers in the AH109A and uptake ratios of AH109A-to-tissues (with the exception of AH109A-to-bone) gradually increased for 60 min. O-[(11)C]methyl-L-tyrosine was metabolically stable, whereas a negligible low amount of metabolites was observed for O-[(18)F]fluoromethyl-L-tyrosine. Both tracers showed the potential for tumor imaging.