The use of 13C-labeling and nuclear magnetic resonance (NMR) spectroscopy to trace the biotransformation of benzoic acid (BA) to hippuric acid (HA) in the rat has been described. Novel [2,4,6,7-13C4]BA, which was labeled in the specific protonated carbons, was used in order to enhance the sensitivity of 13C NMR detection on the basis of the nuclear Overhauser enhancement and short spin-lattice relaxation time. The urinary excretion of [2,4,6,7-13C4]HA formed from intravenously administered [2,4,6,7-13C4]BA was followed by proton-decoupled 13C NMR spectroscopy (only 10 min accumulation time) without any separation procedures such as extraction and chromatography, using [2-13C]sodium acetate as an internal standard for quantitation. The heights of resonances for C2,6 of [2,4,6,7-13C4]HA and C2 of the internal standard were used to calculate [2,4,6,7-13C4]HA concentration. The lower limit of measurable amounts (ca. 40 nmol) was found to be improved about one order of magnitude over that of the method using commercially available [7-13C]BA. In general, this tracer technique has the potential for wide application to pharmacokinetic research since xenobiotic and endogenous metabolism can be followed by very simple and convenient procedures.