The most primitive heart in the animal kingdom contains the atrial natriuretic peptide hormonal system.

1. The newly described atrial natriuretic peptide hormonal system appears to play an important role in the endocrine control of sodium and water metabolism in human and vertebrate animals, but neither atrial natriuretic factor (ANF, C-terminal amino acids (a.a.) 99-126 a.a. prohormone) nor the rest of the ANF prohormone have ever been demonstrated in the heart of an invertebrate. 2. The present investigation was designed to determine whether the earthworm, Lumbricus terrestis, the first animal in the phylogenic tree with any form of heart, has either ANF and/or the 98 a.a. N-terminus of the ANF prohormone. 3. Both an ANF-like peptide (189 +/- 32 ng/g of tissue) and the N-terminus of the ANF prohormone-like peptide (1985 +/- 27 ng/g of tissue) were present in the earthworm heart at concentrations significantly higher (P less than 0.001) than in rat (Rattus norvegicus) heart ventricles. 4. This newly-described hormonal system, thus, appears to be present in a much larger proportion of the animal kingdom than previously thought, including invertebrates as well as vertebrates.