The menstrual flow of five dysmenorrhoeic women was collected, four cycles with and two cycles without local intrauterine administration of progesterone (Progestasert). The menstrual flow of three eumenorrhoeic women served as control. Menstrual blood was collected by tampons stored in butyl-alcohol saturated with water. Prostaglandin F2alpha (PGF2alpha) was measured after purification and extraction by radioimmunoassay. The PGF2alpha production rate and concentration per ml of menstrual blood from dysmenorrhoeic women differed significantly from those of eumenorrhoeic women. In the treated cycles the PGF2alpha levels were significantly decreased (2 alpha less than or equal to 0,01), corresponding to eumenorrhoeic levels. These values were in accordance with the clinical results and indicate that uterine PG-synthesis is under progesterone control.