PIP: At the University Hospital in Louvain, Belgium, plasma hormonal fluctuations in 6 women, aged 16-46, were studied. Measurements were made during the nonpregnant state and in the 4 women who conceived also in the period around nidation. Measurements during the nonpregnant state yielded expected results: Preovulatory midcycle luteinizing hormone (LH) surge preceded by increase of estrone and estradiol, sudden fall of estrogens and rise of progesterone during or just after LH surge, and secondary estrogen rise together with progesterone peak during the luteal phase accurred. In the women implantation occurred, human chorionic gonadotropin (HCG) started to rise sharply 10 or 11 days after the LH surge. Progesterone and estrogen levels remained high during the implantation period. In the patient who aborted at 17 weeks of pregnancy, the HCG rise was slower and the secondary rise of estradiol was rather limited; progesterone levels, however, appeared to be normal. An estradiol surge may occur just prior to implantation.