Reciprocal responses (ventricular "echoes") were initiated by premature stimuli applied directly to the bundle of His in dog hearts perfused from donor animals. The time relations of the responses in the His bundle and at the atrial margin of the exposed A-V node were studied under a number of conditions. It was found that: 1. The earliest premature response that can be induced in the His bundle is not propagated back to the atrium and does not lead to an echo. 2. There is, in most hearts, a continuous range of H 1 H 2 intervals (the echo "zone") within which reciprocal responses of the His bundle occur. 3. The earliest effective H 2 propagates slowly to the atrium, and the echo returns quickly. 4. The latest effective H 2 responses propagate more quickly to the atria, but return more slowly. 5. The atrium is an essential link in the echo pathway; i.e., the H 2 response must reach and activate the atrium before a return is possible. 6. Direct stimulation of the atrium in advance of the expected arrival of the A 2 response propagated from the His bundle can initiate a response which propagates to the bundle; i.e., two responses can travel in opposite directions on a collision course, yet one of them can complete its journey. 7. The limiting parameter of the return pathway is the refractory period of that portion of the lower node which must fire twice in the echo circuit. 8. The limiting element is not the bundle of His. 9. The limiting element can be discharged by early H 2 responses which fail to reach the atrium. 10. The refractory period of the limiting element is a function of the preceding cycle length. It was also demonstrated that the echo "zone" is shifted to a range of longer H 1 H 2 intervals when the basic driving frequency is increased, or when the vagi are stimulated. Epinephrine shifts the echo zone to a range of briefer H 1 H 2 intervals. It is suggested that echo responses occur when a retrograde impulse is arrested at one site within the node and continues to propagate to the atrium through collateral pathways. If the retrograde propagation time is long enough to permit the recovery of the lower elements of the node, an echo response reaches the His bundle. It was concluded that a self-sustained repetitive circuit may be possible when the round-trip conduction time exceeds the refractory period of the limiting tissue in the node.