Few studies have examined the effects of 17beta-estradiol on parameters of insulin and glucose metabolism. We studied 42 healthy, untreated postmenopausal women seeking relief from menopausal symptoms. They were randomized to receive either oral 17beta-estradiol 2 mg daily combined with sequential oral norethindrone acetate (NETA) 1 mg daily from days 12 to 22, or transdermal 17beta-estradiol 0.05 mg daily combined with sequential oral NETA 1 mg daily from days 17 to 28. Intravenous glucose tolerance tests (IVGTTs) were performed at baseline and after 46 weeks (estrogen-alone phase) and 48 weeks (combined phase) of completed therapy. Mathematical modeling analysis of plasma glucose, insulin, and C-peptide concentration profiles provided measures of insulin resistance, secretion, and elimination. Both types of therapy were associated with a decrease in fasting insulin and glucose levels. Insulin sensitivity was increased by oral estradiol during the estrogen-alone phase but was reversed by the addition of NETA. Transdermal estradiol did not affect insulin sensitivity. Hepatic insulin uptake and insulin secretion were increased with both types of treatment. The oral regimen of estradiol therapy was favorable to both insulin elimination and sensitivity. Transdermal estradiol therapy had relatively few effects on insulin metabolism.