In the midcollicular decerebrate cat we have observed unique respiratory variation in heart rate (respiratory-heart rate relationship, RHRR) in respiratory intervals of 2-7 s or Biot's breathing of paired breaths at 16-s intervals. Cardiac slowing that marked the respiratory segmentation of the heartbeat showed consistent relationship with the breath it preceded by 1 to 5 s. Thus, association of respiration and heartbeat must include synergistically central interrelated origins for respiration cardiac rates constituting the RHRR.