Although sheep have been widely used as models in the study of cardiac physiology, corresponding morphologic and morphometric data are scanty. For meaningful correlation of morphometric data with physiological information, it is desirable that fixation of the heart occur under controlled conditions. This paper describes a technique for in situ, retrograde aortic perfusion fixation of sheep myocardium under conditions of controlled pressure and minimal wastage of fixative. This is achieved by the application of snares around the brachiocephalic trunk and aortic arch, which are tightened at the start of the perfusion. These isolate the ascending aorta and the coronary vasculature from the remainder of the circulation and allow fixation of the whole heart at a controlled pressure. The method produces good fixation and contrast for transmission electron microscopy and is applicable to late-gestation fetuses, lambs, and adult sheep.