The biological effects of a new synthetic progestagen, Org 2969 (13-ethyl-11-methylene-18,19-dinor-17.alpha.-pregn-4-en-20-yn-17-ol) were studied in healthy normally menstruating women. Two of them were given 0.125 mg, 5 0.060 mg and 2 0.030 mg of Org 2969 daily on days 1-20 during 1 menstrual cycle. Serum levels of follicle stimulating hormone, luteinizing hormone, progesterone and estradiol were analyzed on days 8-23 in order to evaluate the function of the hypophyseal-ovarian axis. The serum concentrations of aspartate amino transferase, alanine amino transferase, alkaline phosphatase, .gamma. glutamyl transpeptidase and bilirubin were determined to evaluate possible side effects on liver function on days 8, 15 and 23. Serum cortisol was measured on day 8 and 23. The basal body temperature was recorded daily during the whole cycle, and endometrium biopsies were taken on days 21 or 22 of the cycle. All samples were taken similarly during the treatment cycle and the preceding control cycle. According to the hormone determinations, all the treatment cycles were anovulatory except in 1 woman receiving the lowest dose. The treatment led to decreased spinnbarkeit, arborization and sperm penetration in the cervical mucus. Liver function tests and serum cortisol remained unchanged during the treatment.