This report describes a method for studying the arterial endothelial kinetics in the rat by autoradiography of the luminal surface of the endothelium. The procedure involves stripping off the adventitia, sticking the 'whole' artery on a glass slide, and covering the endothelium (with some layers of the media as a support) with the nuclear emulsion. This method gives endothelial autoradiographies with very good optical quality, so that counting the labelled cells is as easy as in the Häutchen preparations. It also eliminates uncertainties due to the occasional lack of endothelium in some areas and the interference of smooth muscle layers, problems which are frequent in the Häutchen techniques used so far. The present method has been tested in rats fed an atherogenic diet for one day and 1, 2, 3 and 4 weeks and in the corresponding controls.