In this preliminary study, the levels of urinary 6 beta-hydroxycortisol and urinary free cortisol and the 6 beta-hydroxycortisol/free cortisol ratio were determined in normal volunteers and in patients with heterozygous familial hypercholesterolemia before and after Pravastatin administration (10 mg/d for 2 weeks). Urinary 6 beta-hydroxycortisol and 6 beta-hydroxycortisol/free cortisol ratio increased significantly in both groups after Pravastatin administration (P less than 0.05). The percent increase of 6 beta-hydroxycortisol/free cortisol did not differ significantly when the two groups were compared. Our preliminary results suggest that Pravastatin induces hepatic microsomal 6 beta-hydroxylase both in normal volunteers and in patients with heterozygous familial hypercholesterolemia.