Atriopeptin (AP) is a peptide hormone synthesized and secreted by the atria of the heart that participates in the regulation of fluid and electrolyte balance. AP-like materials have been detected immunologically in neurons in the central nervous system of the rat. In this study, we have used a solution hybridization-nuclease protection assay to determine whether the brain of the rat contains RNA coding preproatriopeptin, the atrial biosynthetic precursor of AP, and to study the regional distribution of preproatriopeptin mRNA in the brain. We have found that the brain contains mRNA identical to the atrial messenger RNA for preproatriopeptin. AP mRNA is differentially distributed in the brain; the highest concentration was found in the hypothalamus, followed by the cortex and septum, hippocampus, midbrain, spinal cord, olfactory bulb, striatum, and pons and medulla. Very low levels were found in the cerebellum, while no detectable AP mRNA was observed in retina, anterior pituitary, or rat liver. The presence of AP mRNA in the brain demonstrates that neurons have the capacity to synthesize preproatriopeptin identical to that produced in the heart. Bioactive peptides produced from this precursor may be endogenous central neuromodulators as well as a circulating hormone. © 1988.