Two experimental models are presented with which an insight into the efficiency of the cardio-respiratory synchronizing tendencies can be gained. The one works with varying the respiratory cycles (Part 1), the other with varying the cardiac cycles (Part 2). Model 1 discloses that the efficiency of the linking system must be “remarkably high”. Model 2 allows a more precise statement, namely, that respiration remains in phase despite a 30% time-shift of the cardiac reference point. — Work is in progress to refine and to standardize these models.