A prospective monocentric open-label and single-arm trial was performed in 19 postmenopausal women with diagnosed vaginal atrophy. The aim was to determine the extent of a systemic exposure to estriol (CAS 50-27-1). Administration of estriol containing pessaries was daily for 21 days. In order to establish a pharmacokinetic profile after single as well as multiple vaginal doses of 0.03 mg estriol blood samples were taken after the first vaginal administration as well as at day 21 after the last administration. Moreover, in order to control for accumulation additional blood samples were taken predose at days 6, 11 and 16. RESULTS The initial administration increased the population mean estriol plasma concentration to a maximum of 42.1 pg/ml 1 h after dosing. However, already 12 h after administration the estriol concentration had again dropped below 5 pg/ml (lower limit of quantification) in all patients. Repeated administration did not result in an accumulation of estriol, since 2 h after application of the 21st pessary, the population mean estriol concentration reached a maximum of only 11.9 pg/ml. Moreover, no severe or serious adverse events occurred, and no clinically relevant findings were reported. CONCLUSIONS Single vaginal application of pessaries containing 0.03 mg estriol resulted in a very low systemic bioavailability, which decreased even more after multiple dosing confirming a favourable safety profile of low dose pessaries administered daily over 21 days.