A case of complete atrioventricular block, retrograde atrial activation, and intra-atrial block was reported. Inhomogeneous inhibition of the atrioventricular junction and the so-called summation of impulses may both act together to make retrograde conduction possible when third-degree atrioventricular block was present. In lead 2, P waves were dome-shaped, and in lead V1, P waves were biphasic; narrow and peaked P waves intermittently appeared, denoting third-degree intra-atrial block. The peaked P waves signified that the right atrium was activated while the left atrium was not depolarized.