In studies where (2-13C)acetate is employed as isotopic tracer in vivo, we have reported a selected ion monitoring gas-liquid chromatographic/mass spectrometric method which allows plasma tracer enrichment as well as plasma acetate content to be determined in the same 200-500 microliters sample through the use of methacrylic acid as the assay internal standard. For standard solutions in the range equivalent to plasma acetic acid concentrations of 10-200 microM, assay precision was +/- 4.3%. For plasma samples in the physiological range (approximately 20-300 microM acetate) assay precision averaged better than +/- 5%. The use of the method is illustrated by measuring acetate turnover in a young adult.