Quinestrol (ethinylestradiol‐3‐cyclopentyl ether), a synthetic estrogenic compound with prolonged duration of action, was administered for 6 months to women at a dosage of 0.025 mg. twice daily with a minimum 01 untoward clinical efJects. This dosage produced a number of efJects indicative of or compatible with estrogenic activity, including increased cornification of menopausal vaginal epithelium, a slight rise in serum protein‐bound iodine and creatine, and a decrease in beta lipoprotein cholesterol. Unexplained slight decreases in serum acid phosphatase activity and in cephalin flocculation were recorded, and blood lymphocytes increased transiently. Also during quinestrol therapy ACTH produced a greater increase in plasma ll‐hydroxycorticosteroids at the 30 minute point of the stimulation test, but the values in the 1, 2, and 3 hour plasma sampies were comparable in the control and intratherapy studies. Urinary ll‐desoxycortisol metabolites diminished, but the change was very slight. Glucose‐induced hypophosphatemia was accentuated in nondiabetic individuals, glucose tolerance appeared to improve in patients with diabetes, and in both groups the increases in serum insulin evoked by a glucose load were less during quinestrol therapy. Other indices remained unchanged, including blood and serum solutes, a variety of serum enzymes, hematologic and hepatic indices, plasma ll‐hydroxycorticosteroids, urinary 17‐ketosteroids and Porter‐Silber chromogens, plasma ll‐hydroxycorticosteroid responses to intravenous ACTH (the 30 minute exception has been cited) and to pyrogen, and urinary pressor activity, gonadotrop ins, and total estrogens. The efJects of ll‐beta hydroxylase blockade by metyrapone on urinary steroids were not altered by 6 months of quinestrol therapy.