OBJECTIVE To compare the effects of oral and transdermal hormone therapy (HT) on asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide synthase, in postmenopausal women. METHODS In a multicentre, placebo-controlled, double-blind study, 152 hysterectomized healthy women were randomized to receive daily transdermal 17beta-oestradiol (tE2, n = 33), or oral micronized 17beta-oestradiol either unopposed (oE2, n = 37), or continuous combined with gestodene (oE2 + G, n = 33), or placebo (n = 49) for 13, 28-day treatment cycles. Plasma concentrations of ADMA, arginine and symmetric dimethylarginine (SDMA) were measured at baseline and in treatment cycles 4 and 13 with a high-performance liquid chromatography method. RESULTS After 13 cycles all active treatment groups showed a significant reduction in ADMA compared with placebo: tE2, -4.0% (95% CI: -7.5 to -0.6%); oE2, -7.7% (95% CI: -10.9 to -4.4%) and oE2 + G, -7.5% (95% CI: -10.8 to -4.3%). ancova showed a significantly larger reduction in the oral groups compared with the transdermal group (tE2 vs. oE2 and tE2 vs. oE2 + G, both P < 0.01). Oral, but not transdermal treatment, significantly reduced arginine compared with placebo. All active treatments reduced SDMA; however, this was only statistically significant in the oE2 group. CONCLUSIONS Reduction of ADMA was more pronounced after oral than after tE2 administration. Adding gestodene to oral 17beta-oestradiol did not alter the reduction of ADMA. The clinical implications of these findings remain uncertain; however, the decrease of ADMA by 17beta-oestradiol could be a key phenomenon in the modulation of nitric oxide synthesis by postmenopausal HT.