To clarify the effects of female sex hormones on ANF gene expression, emasculated and intact female rats were subcutaneously injected with estradiol (E2), progesterone (P) and olive oil solvent (O) respectively, once a day for 7 days. The relative rANF-mRNA contents of rat atria were measured by molecular hybridization, and rANF-cDNA was labeled with alpha-32P as probe. The groups and hormone doses per 100 g body weight were: (1) sham, 0.2 ml O; (2) rats were bilaterally ovariectomized(OVX), 0.2 ml O; (3) OVX, 80 ng E2; (4) OVX, 80 micrograms E2; (5) OVX, 0.2 mg P; (6) OVX, 2 mg P; (7) OVX, 80 ng E2, 0.2 mg P. The results showed that the rANF-mRNA of group (2) was the lowest, it was 75% of that in group (1), while the rANF-mRNA levels in groups (3)-(7) were 1.1, 2.0, 1.9, 2.3 and 2.1 folds of that in group (2) respectively. The results revealed that E2 and P increased ANF gene expression. Their effects were associated with the dosage. The two may have synergistic actions. Physiological amount of E2 and P may maintain suitable level of rANF-mRNA. This experiment suggested that female sex hormone may have dual effects in body fluid balance.