The concentrations of atrial natriuretic peptide (ANP) and its presumed second messenger, cyclic guanosine-3’,5’-monophosphate (cGMP) were determined in maternal plasma and amniotic fluid. Samples were obtained from normal and pathological pregnancies revealing hydramnios or severe Rh incompatibility between week 16 of gestation and delivery. For analysis of ANP and cGMP, radioimmunoassays were used. ANP and cGMP concentrations in maternal plasma did not differ in normal and pathological pregnancies. In amniotic fluid, we found ANP levels of about 120 pg/ml at 16 weeks of gestation which then decreased below the detection limit of 15.6 pg/ml. In contrast, cGMP levels were low at the beginning of pregnancy (4pmol/ml) and rose significantly (14 pmol/ml), at the end of pregnancy. Pregnancies revealing severe Rh incompatibility exhibited the same levels as normal pregnancies when regular prenatal transfusions resulted in sufficient blood volume substitution. ANP, a volume-homeostasis-regulating hormone, is already produced in early pregnancy. The meaning of the presumed second messenger, cGMP, requires further investigation.