Synthesis and secretion of atrial natriuretic peptide (ANP) is not confined to the heart, but also present in other tissues. ANP is known to affect steroidogenesis in the ovary. To assess the possibilities that (a) the human ovary is a source of ANP secretion as well, and (b) ovarian ANP stimulates progesterone secretion, we investigated ANP as well as progesterone concentrations in preovulatory ovarian follicular fluid from women in an in vitro fertilization program. In all women detectable ANP immunoreactivity was found in ovarian follicular fluid (range 3.3–60.1 pg/ml). Follicular fluid concentrations were low in all but 1 woman who demonstrated a higher ANP level in follicular fluid than in plasma. Follicular ANP and progesterone concentrations were not significantly correlated. These preliminary results suggest that the ovary could well be a site of ANP secretion. If and how ovarian ANP activity affects progesterone secretion remains as yet unknown.