The effects of cortisol, its steric analog 11-epicortisol, and lysine vasopressin (LVP) on DNA and RNA synthesis in isolated adrenocorticotropic hormone-secreting human pituitary tumor cells obtained by transsphenoidal surgery were studied using [3H]thymidine incorporation in DNA and [3H]uridine in RNA. Cortisol suppressed RNA and, to a greater extent, DNA synthesis in these cells. This could explain the slow growth of pituitary tumors in patients with Cushing's disease and the rapid growth of Nelson's pituitary tumors after bilateral adrenalectomy. 11-Epicortisol also suppressed RNA synthesis, but it had a stimulatory effect on DNA synthesis, which indicates a high specificity of glucocorticoid receptors. When applied together with cortisol, 11-epicortisol antagonized the suppressive effects of cortisol on DNA synthesis. Although LVP stimulated RNA synthesis, it suppressed DNA synthesis in most of the tumor cells.