Subderman estradiol (E2) pellets (25 mg) were inserted immediately after hysterectomy and oophorectomy in 22 menstruating women, ages 29 to 50 years. Serum samples were obtained daily for 7 days, weekly for 4 weeks, and at monthly intervals for 6 months. Although there was significant variation between patients, E2 levels remained within the follicular phase range, averaging 50 to 70 pg/ml for 3 months, and then slowly declining to a mean of 37 pg/ml at 6 months, when new pellets were inserted. Over the entire study period, the E2:estrone (E1) ratio was greater than unity. Subdermal E2 pellets limited the rise in luteinzing hormone (LH) and follicle-stimulating hormone (FSH) after gonadectomy and the levels of LH and FSH 6 months after the insertion of E2 pellets were significantly lower (p < 0.01) than in 20 postmenopausal women who had undergone oophorectomy and whose serum E2 levels were less than 20 pg/ml. Serum corticosteroid binding globulin-binding capacity (CBG-BC) and serum testosterone-estradiol binding globulin-binding capacity (TeBG-BC) remained unchanged with E2 pellets. Although high-density lipoprotein-cholesterol increased significantly (p < 0.05), low-density lipoprotein-cholesterol, total cholesterol, and triglycerides were unaffected, except for a rise in triglycerides in three older women with diabetes mellitus and hypertension. There were no complaints of severe hot flushes. Women who had vasomotor symptaoms had mild or moderate flushes that occurred at 5 or 6 months after replacements of the pellets. Thus, E2 pellets are an effective form of parenteral estrogen replacement therapy and offer both practical and theorteical advantages over other forms of estrogen.