Part of the cardioprotective effect of postmenopausal oestrogen replacement therapy has been attributed to arterial vasodilation. This effect is partially reversed in the uterine artery by the addition of a progestagen. This study was designed to compare the effects of the C21 progestagen, dydrogesterone and the C19 testosterone derivative, norethisterone on the carotid artery pulsatility index (PI) (thought to represent distal impedance to flow) using a randomized double blind cross-over trial. The addition of progestagen resulted in a significant increase in the carotid artery PI from a median value of 1.67 during the oestrogen only phase to 1.77 (P = 0.02) during the combined phase. This trend was seen with both dydrogesterone and norethisterone, but there was no significant difference in the size of the effect caused by either progestagen. The addition of cyclical progestagen to ERT partially antagonizes the reduction in the carotid artery PI.