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.