Atriopeptin is a 28 amino acid peptide synthesized primarily in cardiac atria. Atriopeptin acts in opposition to angiotensin II in regulating renal, hemodynamic, and endocrine function. Atriopeptin is released in response to the increased pressure and mechanical stretch of the right atrium due to blood volume overload. Atriopeptin then acts at the nephron to increase salt and water excretion, lowering blood volume and blood pressure. The effects of atriopeptin III (AP III) on in vitro prepared afferent (AA) and efferent arterioles (EA) from rat kidneys were tested in a system in which lumen diameter could be measured. AP III (10(-13)-10(-7) M) had no effect on lumen diameter of AA that were not preconstricted.