Abstract The action of d-thyroxine (DT 4; 0.625 mg/day) and l-thyroxine (LT 4; 0.0625 mg/day) injections and propylthiouracil (PTU; 0.3% of diet) feeding on serum and liver cholesterol and triglyceride levels, 7α-hydroxylation of cholesterol by liver microsomal preparations, and absorption of cholesterol were investigated in the rat. Serum and liver cholesterol and triglyceride levels were not significantly altered. DT 4 administration resulted in significantly higher levels of cholesterol hydroxylation per mg microsomal protein, but hydroxylation was not different from the control when calculated on a total liver basis. PTU feeding lowered hydroxylation per mg protein and per liver. Cholesterol absorption was increased in hypothyroid (PTU fed) rats and decreased in hyperthyroid (T 4 treated) rats. Radioactive acidic fecal steroid excretion was changed very little by any of the treatments, but excretion of radioactive neutral fecal steroids was much higher in DT 4- and LT 4-treated, and much lower in PTU-treated animals. These results indicate that the thyroid hormones mainly affect neutral steroid metabolism.