The potent luteinizing hormone-releasing hormone (LRH) agonist D-Ser(TBU)6-EA10-LRH was administered in a daily subcutaneous dose of 5 or 25 microgram for 1, 3 or 5 consecutive days to twelve regularly menstruating women in the early follicular phase of the menstrual cycle in an attempt to disturb follicular maturation and induce luteolysis. The treatment was monitored by clinical examinations, basal body temperature recordings, bleeding patterns and frequently taken peripheral venous blood samples for analyses of gonadotropins and ovarian steroids. The luteal phase during the LRH agonist treatment cycles did not differ in length from that of the control cycles before and after the treatment (p greater than 0.05). The maximal progesterone concentration during the luteal phase exceeded 32 nmol/l in all but one of the treatment cycles. The follicular phase of the treatment cycle was prolonged in comparison with that of the control cycles (p less than 0.01). Thus, administration of high doses of a superactive LRH agonist during the early follicular phase of the menstrual cycle prolonged the follicular phase and postponed ovulation but did not interfere with corpus luteum function in normally ovulating women.