A group of 22 women with anovulatory cycles was treated with tamoxifen in 46 cycles. We elucidated daily hormone profiles in 5 of the 33 tamoxifen-induced ovulatory cycles in order to make a comparison with those found in 5 normal cycles. Serum follicle-stimulating hormone and luteinizing hormone (LH) rose during or a few days after the tamoxifen therapy, and thereafter serum estradiol levels rose gradually over the normal ranges during the follicular phase and reached an extremely high preovulatory peak which triggered an LH surge. This increased estradiol production exerted its usual negative feedback on LH secretion. Following ovulation, serum progesterone rose and fell in a manner similar to that in the normal cycles, with occasional values that exceeded the normal range. The endocrine significance of these findings is discussed.