Atrial natriuretic peptide (ANP) is a vasoactive molecule with a potential role in modulating uteroplacental blood flow. We examined the expression of ANP in the third-trimester fetomaternal interface by immunohistochemistry performed on sections of placental tissue as well as by reverse transcriptase-polymerase chain reaction (RT-PCR) and RIA performed on isolated, purified term trophoblasts. Results from immunohistochemical studies on ten normal term placental samples revealed labeling of extravillous interstitial and cytotrophoblastic shell trophoblasts. The trophoblastic identity of the cells was determined by labeling of adjacent serial sections with anti-cytokeratin antibody. Labeling of decidual tissue was also observed in some samples (3 of 10 samples), whereas villous trophoblastic tissue showed no labeling. RT-PCR analysis of RNA extracted from isolated third-trimester trophoblasts revealed the presence of a 0.45-kb band corresponding to ANP mRNA. Sequencing of the amplified cDNA confirmed its identity. RIA on lysates of isolated trophoblasts revealed the presence of ANP at a level of 41 pg ± 8 pg/10⁷ cells (n = 4; range = 16.8−50.4 pg/10⁷ cells). These results indicate that placental cytotrophoblastic tissue is a source of ANR This molecule might play a role in the regulation of uteroplacental and/or fetoplacental blood flow.