Based on the new description of AV-nodal conductivity from our laboratory, we developed an equation which describes AV-nodal conduction time in the dog heart at all coupling intervals. This allows us to predict the functional refractory period (FRP) of the AV-node based on the AV-nodal conduction times of only five extra stimuli having relatively long coupling intervals. The equation predicted the FRP within acceptable limits when compared to values obtained by the classical technique in which the full range of extrasystolic intervals must be tested. Changes in FRP after the administration of antiarrhythmic drugs (lidocaine and quinidine) or due to changes in heart rate were predicted accurately. The ease and accuracy of the present method could allow determination of FRP in a short time.