Morphologically the canine cranial cruciate ligament can be divided into a cranio-medial and caudo-lateral component which perform reciprocal functions at all angles of flexion of the stifle joint. Histologically the main constituents of these two components are bundles of longitudinally orientated collagen fibre. The results of the study of the effect of partial and total sectioning of the cranial cruciate ligament on the "anterior-draw" movement implied that the relatively minor degree of movement, elicited following sectioning of either of the components of the ligament, would not be detected under clinical conditions. For joint instability to be clinically detectable most of the ligament must have ruptured, or the intact portion must have undergone degenerative or disruptive changes.