In isolated preparations of the simian (Macaca fascicularis) heart, we studied the activation pattern within the sinoatrial node, using the conventional microelectrode technique. After electrophysiological experiments we subjected three preparations to a correlative light microscopical and two to an electron microscopical investigation. The sinoatrial node of the Macaca fascicularis is characterized by unifocal impulse generation. The impulse is propagated preferentially in an oblique direction towards the inferior vena cava, which is a unique direction compared to all other mammals studied so far. Possible consequences for A-V nodal input are discussed. Conduction block was seen in an oblique superior direction towards the atrial septum. In this small zone of blocked conduction double component action potentials could be recorded. The morphology of the sinoatrial node of the Macaca fascicularis is essentially the same as found in other mammals.