In young women aged 22-34 years the hormone profile under continuous therapy with 0.5 mg lynestrenol/day have been investigated. Estradiol, progesterone and FSH values have been determined in plasma by the RIA or protein-binding method, whereas LH excretion was daily measured in the urine starting on the 5th day of cycle. The low-dose-gestagen therapy shows all possible hormonal patterns ranging from the ovulatory to anovulatory one. The estradiol-17 B values in half of the patients were higher than in the cycle preceding treatment. In the first few treatment cycles all women showed markedly decreased progesterone values. However, this defect seems to recover and return to normal in later cycles. In order to be able to further explain the alterations occurring during the luteal phase under the treatment with 0.5 mg/day of lynestrenol, ovarian tissues wedge resection of women undergoing gynecological surgery has been examined macroscopically and microscopically. At the same time the hormonal profile has been determined in plasma. These investigations revealed that the morphological alterations in the ovary proceed in an unchanged way under the mentioned treatment. Histologically it could be demonstrated that ovulation is delayed by several days. The histological findings of tubal tissue (from patients who underwent sterilization) showed that the secretional activity of the tubal epithelium is considerably decreased and ciliogenesis increased. In our clinical study neither tablet nor patient failures occurred. With the exception of cycle- and tempo-disturbances no severe adverse reactions were reported. Breakthrough bleedings and spotting as well as amenorrhea in most cases manifested already in the first treatment cycles. It is important that the daily tablet is taken at midday because the maximum effect on the cervical mucus of the mini-pill therapy is most pronounced 4-6 hours after tablet intake. Especially in young patients it is recommended to get written reports on tablet intake. The strict following of these directions mainly guarantees a successful mini-pill therapy.