Previous studies have shown that oral, but not transdermal, administration of estrogen stimulates GH secretion in postmenopausal women. Because GH impairs insulin action, the impact of estrogen replacement therapy on carbohydrate metabolism may be influenced by the route of administration. The aim of this study was to prospectively compare the effects of oral and transdermal estrogen replacement on glucose tolerance and insulin sensitivity in postmenopausal women. In an open label, randomized, cross-over study, nine postmenopausal women were randomized to transdermal estrogen patches (Estraderm-TTS 100) or oral conjugated estrogen (Premarin, 1.25 mg) daily for 12 weeks and then crossed over to receive the alternative treatment for a further 12 weeks. An oral glucose tolerance test and hyperinsulinemic euglycemic clamp (HEC) were performed before treatment and at the end of 10 weeks of treatment. Oral and transdermal estrogen both significantly reduced LH to the same degree. Mean GH did not significantly change with transdermal estrogen, but rose significantly during oral estrogen therapy. Peak and mean glucose and insulin levels during the oral glucose tolerance test were not altered by estrogen therapy and were not significantly different between treatments. Mean glucose and insulin levels were maintained at an identical level during the HEC performed at pretreatment and during estrogen therapy. The mean glucose infusion rate required to maintain euglycemia during the HEC (mean +/- SEM, pretreatment, 40.4 +/- 4.8 mumol/kg.min) was unaltered by oral (39.8 +/- 4.6 mumol/kg.min) or transdermal estrogen treatment (42.1 +/- 4.2 mumol/kg.min). However, during the transdermal estrogen phase (60 +/- 10 mumol/L), the mean nonesterified free fatty acid concentration was suppressed to a significantly lower level during the HEC than during the oral estrogen phase (120 +/- 20 mumol/L; P < 0.05). We conclude that compared to the oral route, transdermal estrogen therapy is associated with a slight, but significant, improvement of insulin action on lipid metabolism. However, in the short term, the route of estrogen replacement therapy does not have a major impact on glucose metabolism in postmenopausal women.