The possible polymorphism of the glucuronidation reaction in man has been investigated using two hypolipidaemic compounds, fenofibrate and clofibrate, as the test probes. The formation of fenofibryl and clofibryl glucuronides was identified by their susceptibility to hydrolyses by beta-glucuronidase. The urinary excretion of the glucuronides was measured in 72 healthy volunteers after a single dose of fenofibrate, and in 104 subjects given a single dose of clofibric acid. Fenofibrate was excreted at a lower rate than clofibrate, since 13.94% and 26.55% of the doses of fenofibrate and clofibrate respectively, were recovered in urine in 8 h. Correlation analysis indicated that sex and body mass index significantly influenced the formation of fenofibryl glucuronide, whereas age and oral contraceptives affected the excretion of clofibryl acid glucuronide. The 8-hour urinary excretion patterns of clofibryl glucuronide and of clofibric acid presented a Gaussian distribution, whereas those of fenofibryl glucuronide and fenofibric acid showed 2 populations. When the metabolic ratio free fenofibric acid/glucuronide was considered, 84.7% of subjects presented the ratio 0.147, and 15.3% had the 3-fold higher ratio of 0.421. The study has shown, in the human population studied, that the glucuronidation of fenofibric acid but not that of clofibric acid may present a polymorphism.