We studied antegrade concealed conduction of atrial extrastimuli (A2) that blocked in the atrioventricular (AV) node in eight subjects, using a third extrastimulus (A3), coupled at decreasing coupling intervals to A2. Three A1-A2 intervals were tested in each subject: late (just shorter than AV nodal effective refractory period), intermediate, and early (just longer than atrial functional refractory period). The curves relating the following variables were constructed for each A2: A1-A3, H1-H3 and A2-A3, A3-H3. The former was compared to the control A1-A2, H1-H2 curve. Concealment of A2 was demonstrated in all eight subjects at the three tested values of A1-A2. The A2-A3, A3-H3 curve allowed analysis of AV nodal conduction time (A3-H3) and AV nodal recovery time (defined as the shortest A2-A3 at which the impulse conducted to the His bundle) at identical values of A2-A3. In all subjects the timing of blocked A2 had minimal effect on both AV nodal conduction time and recovery time. In five of the eight subjects a late A2 sporadically conducted to the His bundle. Conduction of A2 to the His bundle resulted in marked lengthening of both AV nodal conduction and recovery times. Concealed conduction of A2 was always demonstrated, but the degree of concealment was relatively fixed, whether A2 was an early, intermediate, or late blocked premature beat. Slow conduction of A2 had a much greater effect than concealment of A2 on subsequent impulse conduction.