The sinu-atrial node (SAN) enables the heart to pulsate continuously by producing impulses. As the histological information about the SAN of ovine fetus is basically needed in various fields of veterinary sciences, such as histology, anatomy, physiology, pathology and cardiology, the present study was planned to provide this basic information in this area. A detailed study was conducted on the SAN of two and four month fetuses (n= 4). Its shape was relatively cone shaped. The dimensions of the node in four month fetuses were 2800 µm in length, 420 µm in width and 500 µm in thickness, where as, in two month fetuses it measured 440 µm in length, 284 µm in width and 130 µm in thickness. The cells of the ovine fetus were occurred as strands of cells. There were color differences between those and the ordinary myocardial cells of the right atrium. This difference was greater in four months old. Also, the amount of the collagen frame and nerve supply of the node was greater in four months fetuses. In general, the percentage of “P” cells in the SAN of ovine fetus was high and the number of these cells was higher in two month fetuses compared with four months old. There were one or some arterioles near the periphery of the cranial end or at the interatrial septum, which supply it.

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