In 5 newborn infants with severe valvular aortic stenosis, left ventricular angiocardiography revealed conspicuous pathologic-morphologic findings consisting of a fine network of contrast medium opacification in the region of the posterior wall. Subsequently performed histologic examination of tissue removed from the involved regions showed, in addition to endocardial fibroelastosis, two structures which were assumed to be the morphologic substrates for the unusual distribution of contrast medium. On the one hand, antler-like spaces with a well-demarcated lining of fibroelastic tissue, similar to the thickened endocardium, were found in the inner half of the left ventricular wall connected to the lumen of the left ventricle through bottle-necked communications. On the other hand, oval or oblong lumina, substantially larger in caliber than capillaries but showing only a single endothelial layer could be demonstrated. With the sectioning technique employed, anastomoses of the latter spaces with the left ventricle or the previously mentioned system of spaces could not be established. The cause of the development or maintenance of these sinusoidal spaces may be attributable to the elevated diastolic left ventricular pressure, while the elevated systolic pressure may have been a decisive etiologic factor for the endocardial fibroelastosis. Whether the intratrabecular spaces represent persistent embryonic myocardial sinusoids cannot be differentiated unequivocally. In support of the stated contentions, however, are other results indicating that the extent of structural changes in the ventricular wall increases proportionately with the duration of existence of the obstructive lesion in the embryonic period.