THE LEAFLETS of the cardiac valves are generally regarded as collagen-reinforced endothelial reduplications which move passively in response to the development of transvalvar pressure gradients during rhythmic contraction of the heart.1Success with valve prostheses has reinforced the view that a passive contribution to undirectional blood flow constitutes the only significant biological role of the valves. Despite the well-documented observation that valves may contain neural and muscular structures,2,3this fact is not widely appreciated. Moreover, preoccupation with the dynamic aspects of atrial and ventricular muscle masses has diverted attention from the possible active role of the valves in cardiovascular regulation. We have begun a reassessment of the basic mechanisms of valve performance by analysis of cardiac valve structure in the dog. Methods Leaflets from all four valves in 15 dogs were excised, fixed, and prepared for study by light and electron microscopy. Valves fixed in formaldehyde solution (formalin)