Atrial natriuretic peptide (ANP), a 28amino acid peptide, is produced and secreted by cardiac atriocytes to modulate cardiovascular and renal functions. We report here the production of ANP-(5-28) and its 15K mol wt (M) presumptive precursors by cytotrophoblasts of rat placentae. Placental tissues were collected from Sprague-Dawley fetal rats on days 12, 16, 18, and 20 of gestation and acid extracted for immunoreactive (ir) ANP assay. The contents of placental irANP increased over the course of fetal growth, with the highest amount (mean f SE, 1083 + 125 pg/tissue; n = 7) found near term. Sephadex G-50 gel chromatographic profiles of the placental extract revealed a major peak of irANP coeluted with the 3K M, of synthetic rat (r) ANP- (l-28) and a minor peak in the position consistent with that of 15K M,. HPLC analysis of the 3K M, species showed a single peak of immunoreactivity, which eluted with a retention time similar to that of rANP-(5-28). In placental sections, irANP and pro-ANP mRNA were localized by immunoperoxidase staining and calorimetric in situ hybridization in a subpopulation of placental cytotrophoblasts, but not in syncytiotrophoblasts or chorionic cells. Northern blot analysis showed a single band of pro-ANP mRNA in rat placental tissues similar in size to that found in the heart (-0.85 kilobases), with the highest level of pro-ANP mRNA signal detected in the placentae of 16-day gestation fetuses. Our findings suggest that ANP is expressed and produced by a small population of rat placental cytotrophoblasts and that the 15K M, precursor peptide is extensively processed into the Nterminal- truncated form of ANP-(5-28) in the tissue.