1. (1) In cats under pentobarbital sodium anesthesia the medulla was searched with indium or tungsten microelectrodes with tips of 4-6 μm for neural discharges with cardiac periodicity. 2. (2) Sixty-seven penetrations were made in the vicinity of the tractus solitarius and solitary complex. Heart-related activity was encountered in 62% of the tracts. In all but 3 instances the activity was located in the posterior two-thirds of the solitary complex, as demonstrated by histological examination. One unit was isolated in nucleus ambiguus, one in the paramedian reticular nucleus and one in the nucleus gracilis. The latter appeared to be related to receptors in the chest wall stimulated by the cardiac impulse. 3. (3) Units encountered varied in the pattern of their discharge and in time relation to the electrocardiogram. Some appeared to be primary afferent discharges; others were attributed to activity of medullary neurons. 4. (4) Respiration had a clear relation to the activity of nearly all units isolated, and was particularly studied with a LINC computer for two units. The great value of computer analysis in studies of this kind was demonstrated.