Studies were made of the effects of temporary occlusion of a coronary artery on the activity of nonmedullated afferent vagal fibers from the left ventricle of 15 cats. Recordings were made from 14 single fibers and 4 filaments with 2 to 3 fibers dissected from the right vagus. Their normal activity averaged 1.8 impulses/sec, and they were identified by an increase in activity to 4 to 17 impulses/sec during a brief occlusion of the aorta and by a conduction velocity of 0.6 to 1.3 m/sec (mean 0.9). The receptors studied were distributed throughout the ventricle, as determined by mechanical probing of the nonbeating heart at the end of the experiments. With a 1 to 1.5 minute occlusion of a coronary artery, the activity of receptors within the area supplied by that artery increased in concert with the systolic bulging of the ischemic area. The initial increase in activity occurred during systole, suggesting that the receptors were activated mechanically rather than by chemical changes in the ischemic zone. The activity soon became continuous, reaching a maximum of 15.4 impulses/sec (mean value). The maximal activity was not sustained; during occlusion of up to 40 minutes it decreased to 5 impulses/sec (mean value after 5 to 10 minutes. The decrease may result from anoxia since three of four tested receptors showed an increase in activity after release of occlusion.