Although cultures of neonatal rat atria and ventricles have been widely used to study ANP biosynthesis and secretion, little is known regarding the circulating form of ANP in neonatal animals. To begin to address this issue, we have developed a method for perfusing isolated neonatal rat hearts. Reversed phase-HPLC analysis of the heart effluents coupled with ANP RIA demonstrated that the predominant form of ANP released was chromatographically identical to ANP(99-126). Size exclusion-HPLC confirmed that the secreted ANP was indistinguishable from ANP(99-126). This demonstrated that the neonatal rat heart can efficiently generate and secrete a peptide similar to the circulating form of ANP found in adult rats, and further justifies the use of neonatal rat atria as a source of primary cells for studies of ANP biosynthesis and secretion.