A study was made of the effect of hypocholesterolemia induced by 3-day administration of 17 alpha-ethinyl estradiol on the activity of lysosomal and cytoplasmatic cholesterol esterases, acyl-CoA: cholesterol acyltransferase and on the free and esterified cholesterol concentrations in the rat adrenals. A decrease in the content of esterified cholesterol in the adrenal tissue was accompanied by a decrease in the activity of acyl-CoA: cholesterol acyltransferase and by an increase in the activity of lysosomal cholesterol esterase. The activity of cytoplasmatic cholesterol esterase was not changed significantly. The data obtained were discussed with relation to the synthesis of steroid hormones in the rat adrenals.