In view of the increasing use of oestrogen-progestogen therapy in post-menopausal women the availability of possible biochemical marker of endometrial response to progestogen would be highly desirable. Placental protein 14 (PP14) has recently been shown to be excreted from the endometrium during the secretory phase of the menstrual cycle. In the present study we examined the concentration of serum PP14 (S-PP14) in 15 pre-menopausal women during the menstrual cycle and in 30 early post-menopausal women receiving three different doses of 17 beta-oestradiol (E2), cyclically combined with norethisterone acetate (NETA). S-PP14 levels peaked on day 1 following the onset of menstrual bleeding and on day 7 during NETA administration. It was possible to estimate the area under the S-PP14 curve reliably from one or two blood samples taken at optimal times. In the post-menopausal women, S-PP14 varied in a highly significant dose-dependent manner, i.e., the higher the E2 dose (in relation to the fixed NETA dose) the higher the S-PP14 response was found to be. We suggest that S-PP14 measurements might reflect the quantitative development of the endometrium in the secretory phase in both pre-menopausal and post-menopausal women.