The response of 14 atrial receptors with nonmedullated vagal afferents to changes in atrial pressure was determined in anesthetized thoracotomized cats. Recordings were made from 11 single fibers and three two-fiber filaments. Mean conduction velocity was 0.9 m/sec (range, 0.35 to 2.2 m/sec). During the control period, either no activity was detected or there was a sparse discharge (mean, 1.4 impulses/sec), which occasionally was related to the a or v waves. As atrial pressure was increased by transfusion or by occlusion of the aortic, pulmonary arterial, mitral, and tricuspid orifices, an increased rate of firing occurred, often related to the atrial v wave. The threshold for individual receptors was between 2 and 3 mm Hg (mean pressure) in the right atrium, and 5 and 12 mm Hg in the left atrium. The maximal firing rate was 5-11 impulses/sec for right atrial and 10-20 impulses/sec for left atrial receptors. The receptors were localized by probing the opened heart, and were identified in both atria, in the interatrial septum, and in the atrial-venous junctions. Thus receptors connected to vagal C fibers are present throughout both atria and are activated by moderate changes in pressure.