An RIA for a new progestin, ST-1435, was developed. Plasma concentrations of this steroid were measured in three women having three subcutaneous capsules, each containing 40 mg of ST-1435. Pituitary and ovarian functions were assessed by measuring plasma levels of LH, FSH, estradiol and progesterone. Plasma profiles of ST-1435 were characterized by high and irregular peaks all the way through the study period of 7-9 months. Concentrations below 100 pg/ml were not observed, whereas the highest peaks reached concentrations of 10.1 ng/ml. Plasma estradiol concentrations remained uniformly suppressed below 100 pg/ml, thus indicating a suppression in follicle development. As judged by constantly suppressed progesterone concentrations, no ovulations occurred during the treatment. Plasma FSH concentrations remained at the upper limit of the follicular phase, and those of LH at the lower limit of the luteal phase of the normal menstrual cycle. No midcycle gonadotropin peaks were observed.