Plasma concentrations of atrial natriuretic factor were determined by radioimmunoassay in 16 human fetuses of between 19 and 38 weeks' gestation. Fifteen fetuses had varying degrees of anaemia as a result of Rh isoimmunisation, and one fetus was normal. Eight fetuses had ultrasonographic evidence of severe hydrops fetalis and an additional three fetuses had mild hydrops. Severely hydropic fetuses were more anaemic and immature than those with mild or no hydrops. Among fetuses from which samples were taken before in utero transfusion, concentrations of atrial natriuretic factor were higher in those with severe hydrops than in the other groups. An inverse relationship between the haemoglobin concentration and that of atrial natriuretic factor was found. In four fetuses in which severe hydrops resolved after intravascular transfusions in utero, there were significant decreases in plasma atrial natriuretic factor concentrations; in the fifth fetus the decrease was less pronounced. Raised concentrations of atrial natriuretic factor in fetuses with severe anaemia and hydrops may be the result of atrial natriuretic factor release induced by hypoxia.