FURUHASHI, N., TSUJIEI, M., KIMURA, H., YAJIMA, A., NAGAE, H. and KIMURA, C. Maternal and Fetal Atrial Natriuretic Peptide Levels, Maternal Plasma Renin Activity, Angiotensin II, Prostacyclin and Thromboxane A₂ Levels in Normal and Preeclamptic Pregnancies. Tohoku J. Exp. Med., 1991, 165 (2), 79-86-To clarify the possible role of elevated atrial natriuretic peptide (ANP) in the pathophysiology of preeclapmsia, we measured ANP, renin activity (PRA), angiotensin II (Ang II), TXB₂ (a stable metabolite of TXA₂) and 6-keto-PGF_1α (a stable end product of PGI₂) concentrations in the plasma of 19 normal pregnant women and 35 severe preeclamptic patients at term. Plasma ANP levels in the preeclamptic patients (n=35, 71.5±3.8pg/ml, mean±S.E.) and also umbilical plasma ANP (n=35, 83.0±4.2pg/ml) were significantly (p<0.01) higher than those of normal pregnant women plasma (n=19, 58.7±3.7pg/ml) and umbilical plasma (n=19, 47.6±4.7pg/ml ). There was a significant (p<0.01) positive correlation between maternal ANP levels and fetal ANP levels (n=54, r=0.44). Plasma PRA and 6-keto-PGF_1α levels in preeclampsia were significantly (p<0.05) lower than those of normal pregnancy. The ratio of 6-keto-PGF_1α/TXB₂ in preeclampsia was significantly (p<0.01) lower than that of normal pregnancy as we reported previously. There was no significant correlation between plasma ANP level and plasma PRA, Ang II, plasma TXB₂ and 6-keto-PGF_1α concentrations. Moreover there was no significant correlation between plasma ANP level and the severity of preeclampsia. These data suggest the possibility of a transplacental crossing of ANP secreted by feto-placental unit, which might be, at least in part, responsible for the high ANP levels observed in preeclampsia. The ANP in preeclampsia is not related directly to hypertension, but it may play a substantial role in the regulation or normalization of blood volume and vascular reactivity.