We used the isolated superfused preparations of right atrium with the zone of the sinoatrial node of rabbits to study the effects of histamine on the location of the dominant pacemaker. The preparations were cut into four pieces and electrical activity was recorded in each fragment by means of a surface suction electrode with internal KCl perfusion. Under control conditions, the maximum frequency of monophasic potentials was found in the fragment with the central part of the sinoatrial node. Histamine (10(-5)M) increased spontaneous activity of all fragments. However, the maximum frequency of potentials was recorded in the fragment with the inferior part of the sinoatrial node. This effect of histamine was reversible upon washout. We conclude that histamine activates the latent pacemakers and induces a pacemaker shift towards the inferior part of the sinoatrial node.