In order to determine whether there is a relationship between genetically determined salt-induced hypertension and atrial natriuretic factor (ANF), a radio-immunoassay for ANF was applied to the determination of immunoreactive ANF in plasma, atrium, hypothalamus and pons of Dahl salt-sensitive (S) and -resistant (R) rats which were fed high- or low-salt diet for 7 weeks. A twofold higher concentration of plasma ANF was observed in high-salt S rats, which developed hypertension, compared with low-salt S rats or R rats on high or low salt, which were normotensive. No significant difference was seen in atrial concentrations of ANF between S and R rats. The brain ANF concentration of the high-salt group was lower than that of the low-salt group in both S and R rats. It is proposed that the elevation of plasma ANF in the hypertensive rats may reflect a compensatory mechanism induced by volume expansion in the salt-fed S rats.