The adrenal gland requires a continuous supply of cholesterol for the biosynthesis of adrenal corticosteroids, which can be supplied by low density lipoprotein receptor-mediated uptake or local synthesis. The present study examined whether hypolipidemic therapy with a potent HMG CoA reductase inhibitor, simvastatin, compromises the adrenal response to ACTH stimulation in adult patients with heterozygous familial hypercholesterolemia. The adrenal response to a 36-h continuous ACTH infusion was determined at baseline and after 2 months of simvastatin treatment (40 mg, twice daily) in eight patients. Simvastatin reduced total and low density lipoprotein cholesterol levels by 36% and 45%, respectively. The time course of the increase in serum cortisol concentrations with continuous ACTH infusion was the same before and during simvastatin therapy, as were the rates of urinary excretion of free cortisol, 17-hydroxycorticosteroids, and 17-ketosteroids. Urinary excretion of mevalonate, which correlates with rates of whole body cholesterol synthesis, decreased from 3.8 +/- 0.42 (+/- SEM) mu,ol/24 h at baseline to 2.75 +/- 0.56 on simvastatin; no significant changes were seen in the urinary mevalonate levels before and after simvastatin therapy during ACTH stimulation. We conclude that the hypolipidemic effects of simvastatin in patients with heterozygous familial hypercholesterolemia are paralleled by a decrease in urinary mevalonate, but that the drug does not adversely affect ACTH-stimulated adrenal corticosteroid production.