We prepared 11C-labeled 5-(4-chlorophenyl)-1-(4-methoxyphenyl)-3-(trifluoromethyl)-1H-pyrazole ([11C]1) and 4-[5-(4-methoxyphenyl)-3-trifluoromethyl-1H-pyrazol-1-yl]benzenesulfonamide ([11C]2) for imaging COX-1 and COX-2 isoforms, respectively, by positron emission tomography. [11C]1 and [11C]2 were synthesized in high radiochemical yields by O-[11C]methylation with [11C]methyl triflate in acetone containing an equivalent of NaOH as a base with respect to the phenolic precursors. In vivo evaluation in rats bearing AH109A hepatoma demonstrated minimal specific binding of [11C] to COX-1 in peripheral organs, such as the spleen and small intestine. Carrier-saturable uptake of [11C]2 was found in the spleen, but COX-2-specific binding of [11C]2 was not identifiable in the brain, AH109A hepatoma or other peripheral organs, although ex vivo autoradiography showed regionally different distribution in the brain and AH109A. The results suggest that neither [11C]1 nor [11C]2 is a suitable radioligand for in vivo biomarkers of COX enzymes, mainly because of marked non-specific binding.