We describe unusual responses of the sinus node to programmed atrial stimulation in two asymptomatic cardiac transplant recipients. In one patient the sinoatrial conduction time, calculated using the revised method of Strauss, is extremely short (5 ms), and in the other it is extremely long (460 ms). The various mechanisms that might be involved in these atypical responses to atrial extrastimulation are discussed. These include sinus node suppression, shift of pacemaker, direct stimulation of the sinus node and shortening of the sinus node action potential duration.