Oxidative modification of low-density lipoprotein (LDL) may be atherogenic. We studied the time of onset of LDL oxidation (lag) in 18 postmenopausal women before and after intra-arterial infusion of 17 Beta -oestradiol, after 3 weeks' patch administration in 12 of these women, and 1 month after discontinuation in 10. The lag increased from baseline after acute infusion (from 134 (SD 41) to 167 (36) min, p=0.01) and after the patch (132 (31) to 178 (45) min, p=0.009). After discontinuation of oestradiol, the lag returned to baseline. This study shows an antioxidant effect of physiological levels of 17 Beta -oestradiol, which may contribute to an anti-atherogenic action.