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Prorenin (PR) an inactive high molecular weight form of renin normally circulates in human plasma at a concentration of about 10 times that of active renin (AR) and this proenzyme seems to be linked to the reproductive function. It has been demonstrated that AR and PR are present at high concentrations in follicular fluid when the ovaries are stimulated with gonadotropins and that the PR plasma levels increase steadily after hCG injection with a correlation between blood PR and the number of developing follicles and corpora lutea. From September 1986 we studied the profile of immunoreactive active renin (AR) and prorenin (PR) in plasma during hyperstimulated cycles for IVFET or GIFT. All women were treated with a protocol combining GnRH analog (Decaptyl Ipsen Biotech, Paris France) and human menopausal gonadotropins until injection of 5,000 IU hCG. AR has been assayed in frozen samples by specific immunoradiometry (Renin RIA code 79 795, Pasteur Diagnostic, France) using two complementary monoclonal antibodies. A second assay of total renin was carried out after trypsin activation which revealed the inactive form. Progesterone (P), estradiol (E2) were measured by radioimmuno-assays. During the follicular phase, from day 1 of hMG administration to the day before hCG, no significant difference could be found between two groups, 63 pregnant or 60 nonpregnant cycles matched for age and number of oocytes retrieved, for E2, P, PR and AR. During the periovulatory period (D - 1, Do = day of hCG injection and D 1) no difference could be found for E2, P and PR (tabl. 1). In the 2 groups the mean E2 levels increased after hCG injection, as well as P and PR. But a significant difference appeared for AR which increased in the plasma immediately after hCG administration in the pregnant group whereas it decreased in the non-pregnant group (+2.5 vs -2 pg/ml) the mean variation between Do and D + 1 being significantly different in fertile cycles and in nonfertile cycles.(ABSTRACT TRUNCATED AT 400 WORDS)