Menstrual blood of 5 dysmenorrhoeic women was collected in tampons during two menstrual cycles without and four cycles with intrauterine progesterone treatment (Progestasert). Three eumenorrhoeic women served as controls. Progesterone was assayed radioimmunologically after purification and extraction. Total progesterone excretion in the menstrual blood was the same in eumenorrhoeic and dysmenorrhoeic women without treatment. An increase of menstrual progesterone excretion by the factor 13 was found during the first bleeding after insertion of progesterone T. After 12 months of treatment with Progestasert only 41.7% (2 alpha less than or equal to 0.001) of the initial progesterone amount per menstruation was found. The progesterone concentration per ml menstrual blood had decreased in the same way but was still clearly higher than eumenorrhoeic values. The results indicate that progesterone controls intrauterine prostaglandin F2alpha formation indirectly.