A comparative study of the Haversian architecture was carried out on compact bone derived from the anterior and posterior edges of the diaphysis of horse radius, regions which have different mechanical requirements in vivo. Samples were heat-deproteinated prior to SEM analysis, a treatment which effectively removes cells and vascular structures as well as exposing large areas of the mineralization front along the walls of the haversian canals. Bone subject to tensile stress revealed a prevalent alignment of its collagen fibrils in the stress direction, and the vast majority of its osteons were composed of fibrils running almost parallel and crossing at very acute angles. Bone subject to compressive forces showed either an orthogonal alternation of collagen lamellae or a multidirectional arrangement corresponding to the twisted plywood pattern described by other authors. Our observations substantiate both the classical model of the osteon and the twisted plywood concept, and suggest that osteon ultrastructure is modulated according to biomechanical requirements.