In this study we employed a capillary gas-liquid chromatographic-mass spectrometric (GLC-MS) method to measure the plasma concentrations of 3-hydroxybutyrate (3-OHB) and 3-hydroxyisobutyrate (3-OHIB) in overnight fasted diabetic subjects and in normal subjects. Plasma contents of 3-hydroxybutyrate measured in this fashion were identical to those obtained by enzymatic assay using a commercial preparation of beta-hydroxybutyrate dehydrogenase, indicating no significant contamination of this enzyme preparation with 3-hydroxyisobutyrate dehydrogenase. In normal individuals, plasma 3-OHIB concentration was 21 +/- 2 microM in the overnight fasted state and was higher in diabetic subjects (38 +/- 5 microM) and in subjects fasted for 72 h (97 +/- 4 microM). In the postabsorptive state, 3-OHIB was 33% the concentration of 3-OHB in normals and 17% that of 3-OHB in the diabetics.