To investigate the activation of particulate guanylate cyclase in pregnant women with pregnancy-induced hypertension (PIH), plasma levels of atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and guanosine 3',5'-cyclic phosphate (cGMP) were measured by radioimmunoassays specific to each substance. Ten normal nonpregnant women, and 30 normal pregnant women, 17 pregnant women with mild PIH and 11 pregnant women with severe PIH in the third trimester were included in this retrospective observational study. The diagnosis and classification of hypertension were carried out according to the technical bulletin (No. 91) of the American College of Obstetricians and Gynecologists. In the pregnant women with mild PIH, plasma cGMP levels as well as ANP and BNP levels were significantly (P < 0.05) higher than those in gestational age-matched normal pregnant or nonpregnant women. But in the pregnant women with severe PIH, plasma cGMP levels were significantly lower than those in pregnant women with mild PIH (P < 0.05), although plasma ANP and BNP levels were higher than those in pregnant women with mild PIH.