Scanning electron microscopy of rabbit aorta showed that the intimal folds had a regular, linear pattern in areas of expected laminar flow. In areas of expected vortex formation, however, there was deviation of the fold pattern with the formation of rounded depressions or pits in the intimal surface. Platelet depositions were found in connection with some of the pits. In sections for light microscopy the pits corresponded to small areas of flattened folds of the innermost elastic membranes. It is concluded that the intimal pits probably reflect imprints of vortices of the whirling blood.