The characterization and distribution of non-muscle cells of the adult rat heart were carried out by scanning and transmission electron microscopy. The non-muscle cells were examined during different phases of dissociation of the adult heart into single cell suspension. Non-muscle cells constituted approximately 65--70% of the ventricular cell suspension, while muscle cells constituted approximately 30--35%. Five types of non-muscle cells were observed: (1) endothelial cells, (2) fibroblasts, (3) pericytes, (4) smooth muscle cells, and (5) macrophages. The endothelial cells lining the internal wall of the ventricle possessed different surface morphology and shape than those lined with blood vessels. Fibroblasts were mostly scattered among the cardiac muscle cells and they contained highly developed rough-surfaced endoplasmic reticulum along with other structural features. Pericytes were characteristically observed on the periphery of the blood vessels and they showed structural similarities to the fibroblasts. Smooth muscle cells were relatively fewer than other non-muscle cells and generally served to line the wall of the medium calibre blood vessels of the heart. They were easily identified with their myofilaments. Although macrophages were observed in different regions of the heart, a small number of macrophage-like cells were strongly attached to the surfaces of the cardiac muscle cells. These cells were not released from the cardiac muscle cell surfaces even after prolonged treatment of proteolytic enzymes. Macrophages were mainly identified with their abundant filopodia, lysosomes and lysosomal degradation products. The functional implications of these non-muscle cells in relation to the heart was discussed.