The changes in ovarian levels of immunoreactive atrial natriuretic peptide (irANP) and arginine vasopressin (irAVP) were observed during the estrous cycle of rat. We also demonstrated the synthesis of ovarian ANP. In adult 4-day cycling rats, ovarian level of irANP was found to be the highest on proestrus and was to be the lowest on diestrus. Ovarian irANP level inversely correlated with ovarian level of irAVP. On reverse-phase HPLC, two distinct peaks of ovarian irANP, high and low molecular weight forms, existed in the each stage of the estrous cycle. However, no significant changes in plasma and atrial concentrations of ANP were observed during the cycle. The rat ovary contained mRNA coding for ANP. These data showing the synchronized cyclic change of ovarian irANP and irAVP with the estrous cycle suggest that the ovary locally synthesizes ANP and ovarian ANP may play regulatory roles on the follicular fluid dynamics.