Antisera to atriopeptin III and to a cyanogen bromide fragment of the precursor molecule atriopeptigen were prepared and used to examine the distribution of atriopeptin-like immunoreactive material in the heart and brain of the rat. Granules of this material were seen in myocytes throughout the right and left atria and were densest in the perinuclear region. The distribution of atriopeptin-like immunoreactive material in the heart is consistent with previous reports of atrial secretory granules. In the brain neurons containing the material were observed in the hypothalamus and the pontine tegmentum. Atriopeptin in the brain may serve as a neurotransmitter in neural systems controlling blood volume and composition, the same physiological functions regulated by blood-borne atriopeptin.