L-Gulonic acid, a diastereomer of d-gluconic acid is the one of a metabolite in the urinate cycle, is a substrate of L-Gulonate 3-dehydrogenase (GDH). This enzyme catalyzes the NAD(+)-linked dehydrogenation of L-gulonate into dehydro-L-gulonate. L-Gulonic acid is also a metabolite of the D-glucuronic acid pathway. It was suggested, that the measurement of a spectrum of urinary D-glucuronic acid metabolites might provide a more reliable index for assessment of the induction of hepatic xenobiotic-metabolizing enzyme activities in man than the determination of urinary D-glucaric acid alone.