Right ventricular septomarginal trabeculae ('false-tendons') from puppies, young adult, and older adult dogs were examined by light and electron microscopy. The connective tissue of the trabeculae obtained from the puppies and the young adult dogs had few elastic fibers, but this component was well developed in the connective tissue of the adult dogs. The trabeculae of older dogs also showed scattered foci of extracellular fat droplets, and their junctional regions nearest to the ventricular wall were often heavily laden with fat. The Purkinje cells were uniform in each group, but differed from one group to another: in the puppies they resembled typical conducting cells, being predominantly cuboidal with few, poorly developed myofibrils, whilst in the adults the Purkinje cells resembled working myocytes, being elongated with a well developed complement of longitudinally arranged myofibrils. The cells of all age groups showed a complete absence of T-tubules, simply arranged intercalated discs, and frequent dilatations of the sarcoplasmic reticulum. The cells of older dogs showed separations of the undifferentiated regions of most discs. Membrane degenerations and the presence of numerous fat droplets also were common. The fine structural cytology of Purkinje cells therefore appears to change considerably with age in the right ventricular septomarginal trabeculae ('false-tendons') of the dog heart.