Eleven patients with retrograde gaps by conventional electro-physiological studies were reported. Most of the patients were studied because of arrhythmia. Electrical activity of the His bundle, atrium and routine ECG were recorded simultaneously. The excitability and characteristics in retrograde conduction were observed, using the programmed ventricular stimulus. The results of this study showed that two types of retrograde gaps occurred, Type I: The site of proximal delay was within the His Purkinje system (HPS), and the site of distal block was in the A-V node. Three of the 11 cases were of this type. Type II: Both the site of proximal delay and distal block are within the HPS in eight of the 11 cases. Comparing the data with those from our lab about the gap phenomena during atrioventricular conduction in 10 cases, it was found that in the gap phenomena of A-V conduction, the site of proximal delay is usually within the A-V node, whereas the site of distal block was in the HPS. In the gap phenomena of V-A conduction the site of distal block might be either above or below the His bundle, but the site of proximal delay was only occurred in the HPS. So retrograde gaps were more common than antegrade gaps depending upon the ratio of the functional refractory periods of two horizontal region in the A-V (or V-A) conduction system.