This report demonstrates unusual responses of the sinus rhythm to atrial pacing. The sinus rhythm failed to become manifest when the heart was driven at a rate slower than the inherent sinus rate. Sinus rhythm returned only after termination of underdrive pacing with the recovery time longer than twice the cycle length of the control sinus rhythm. The largest difference between underdrive and sinus cycle lengths measured 600 msec. To the best of our knowledge, underdrive suppression of the sinus rhythm has not been previously reported in man.