To evaluate the refractory periods of the atrioventricular (A-V) conducting system in patients with a short P-R interval and normal QRS complex, 57 patients with a P-R interval of 110 to 280 msec were studied with His bundle recording and premature atrial stimulation at similar cycle lengths of 660 to 720 msec. In 13 patients with a short P-R interval (120 msec or less) the mean value for the functional refractory period of the A-V node was 368 plus or minus 36 msec (standard deviation), which was significantly lower (P smaller than 0.01) than the mean value of 415 plus or minus 50 msec in 36 patients with a normal P-R interval (between 120 and 200 msec) and the mean value of 492 plus or minus 57 msec in 8 patients with an increased P-R interval (200 msec or greater). Patients with a short P-R interval had a mean value for the effective refractory period of the A-V conducting system of 247 plus or minus 26 msec, which was significantly lower (P smaller than 0.01) than the mean values of 297 plus or minus 51 msec in patients with a normal P-R interval and 369 plus or minus 87 msec in patients with an increased P-R interval. The results identify a characteristic conduction abnormality that is compatible with a partial A-V nodal bypass or dual A-V nodal conduction pathways. The relation between the duration of the P-R interval and the regractory period may aid understanding of the clinical significance of the scalar electrocardiogram.