Aim of  the Work: This research was conducted to study the structure of the left AV (mitral) valve in different age groups of albino rat, and also to clarify the structural changes in the valve in elderly that might be a contributing factor in the valvular heart disease. Material and Methods: Twenty- four male albino rats were used and divided into three groups including juvenile (21days), adult (6 - 8 months) and senile (24 months). Animals of all groups were anaesthetized with ether. The thoracic wall was opened and the hearts were extracted and examined by both light and scanning electron microscopes. Results: Examination of the left AV (mitral) valve revealed that the leaflets were consisted of a continuous curtain that showed no commissures or clefts. Two conal papillary muscles (anterior and posterior) served the mitral valve and originated from the heart apex. These papillary muscles were not separated from the ventricular wall but remained in continuity with the trabecular complex. Chordae tendineae radiated out from the apices of the papillary muscles and merged either with the free border or the ventricular surfaces of the leaflets. The surfaces of the leaflet of the left AV valve were invested with a thin endocardial layer continuous with that of the heart chambers. In senile age groups, cracks of the surface of the leaflet with denudation of endocardial cells were noticed exposing the subendocardial collagen matrix. The ventricular side of the leaflets was marked by the presence of wavy collagenous bundles which increased in thickness and compactness with age. The atrial side composed of numerous interstitial cells embedded in a loose network of collagenous fibers. In juvenile group, the majority of the interstitial cells were myofibroblasts, while in adult and senile age groups of rats most of them were fibroblasts. Chondrocytes were observed at the site of attachment of leaflets to the valvular annulus. These cells were clearly more obvious with age. Conclusions: Aging was associated with changes in the left AV valve that might be the causes of regurge of mitral valve in the elderly