A simple and sensitive radioimmunoassay procedure has been developed for the determination of atrial natriuretic factor (ANF) in human plasma. The rabbit antiserum was obtained from a commercial source. ANF was extracted from plasma using an octadecasilyl silica cartridge with a recovery of 78.7%. HPLC of the plasma extract showed the presence of one immunoreactive peak of ANF corresponding to its low molecular weight form. Plasma ANF in humans increased from 8.0 +/- 2.2 in upright position to 20.0 +/- 5.9 fmol/ml (n = 6) in downward position (p less than 0.005).