Oestrogen-containing vaginal rings of various designs have been utilised in hormone replacement therapy. In contrast to the traditional ‘homogeneous’ design, rings designed with a steroid-containing core and outer polymer sheath provide a diffusion-controlled release rate which enables the delivery of low doses of drug. The aim of this investigation was to evaluate in vitro oestradiol release from a ‘core’ designed vaginal ring (Estring®) and furthermore, to establish the in vivo concentration-time course of oestradiol, oestrone and total oestrone (unconjugated plus conjugated) in consecutive applications of such an oestradiol-containing vaginal ring in postmenopausal women. Results indicate that the controlled release design of Estring® produces stable, low systemic plasma concentrations of oestradiol and has an extended time period of release.