Aim Study on the accurence and regluar patterns of atrial natriuretic factor (ANF) in the human heart befor birth. Methods Collection of 65 embryo cares from accidental abortionsor miscarriages or induced labour of the pregrant women with no cardiovascular disease in clinical diaynosio, measurement of the amount of ANF in different locations of the embryocal heart means of radioimmunoassay. Results ANF in the atria appeared in the sixth week ,but in a minute amount, increasing with the growth of the embryo, while the amount of ANF in the ventricals reached a rather higher level in the sixth week, decreasing with the growth of the embryo and then nearly disappeared after a full term. In the right auriclua of the embryos at the same age ANF reached the maximam, with the amount in the right atrium over that in the left, and the amount in the left ventricle over that in the right. Conclusions ANF in the human embryocal atria appeared in the sixth week, increasing with the growth of the embryo while ANF was also found in the ventricles and the interventricular septa, decreasing with the growth of the embryo.