The myo-endothelial area in the coronary artery conduit was described in 3 developmental stages: in fetuses, newborns, and adult dogs. Transmission and scanning electron microscopy were used for the study, and morphometry was used for quantitative evaluation. In all three stages, the internal elastic lamina was found to be fenestrated. Endothelial cells and smooth muscle cells (SMC) approached the fenestrae, and protrusions of one or both cells entered into the fenestrae. In some places contacts between endothelial and SMC were found. The patterns of mutual approaches of smooth muscle and endothelial cells, as well as the entering into the fenestrae were similar in all three stages. The myo-endothelial contacts were counted per 100 μm inner circumference of the coronary artery and the numbers observed, i.e. 5.17 ± 0.50 in fetuses, 1.94 ± 0.17 in newborns and 0.33 ± 0.09 in adult animals, proved clearly that the frequency of myo-endothelial contacts, highest in fetuses, decreases with age. With regard to the dual control of the coronary smooth muscle and/or diameter, it is noteworthy that an opposite trend can be observed in the development of innervation of the coronary artery: the autonomic nerve fibres with varicosities are missing in the coronary wall 1 week before birth, while after birth their number keeps increasing. Remarkable enough is also the difference in distances between the endothelium and SMC on the one hand and nerve varicosities and SMC on the other. The above facts indicate a prevalence of endothelial control of coronary diameter.