In this histological study, the inhibition of continuous oral administration of lynestrenol 5 mg/day on the ovarian follicle and corpus luteum development was found to be extremely pronounced. Known estrogen - producing structures such as antral, mature follicles and corpora lutea were completely absent during treatment. Structures devoid of cystic function however, as persistent and cystic follicles and follicular cysts, seemed not to be influenced in their histological appearance and frequency. As the recovery of the cyclic ovarian function after cessation of the medication compares favorably with the spontaneous cycle, a continuous lynestrenol treatment of 5 mg/day can be regarded as a safe endocrinological regimen.