The development of culture techniques for endothelium of the large vessels has stimulated many studies to understand endothelial functions in normal and pathological conditions. In this report we describe that in primary cultures the mean surface density of pinocytotic vesicles, evaluated by computerized morphometric analysis of endothelial cell plasma-membrane, dramatically decreases with respect to that of the cells immediately detached from the arterial wall (6.7 +/- 1.1 microns2 against 19.5 +/- 2.2 microns2, p less than 0.001). The results are unchanged if the cells are enzymatically or mechanically detached from the vessel wall or from the culture flask. After the first passage, the mean surface density of pinocytotic vesicles decreases further (2.5 +/- 1.3 microns2 p less than 0.01). After the 2nd and the 3rd passages, the morphometrical values of endothelial cell plasma-membrane remain low (1.5 +/- 0.2 microns2; 2.5 +/- 0.2 microns2). When endothelial cultures are employed to study pathological aspects of disease, not only the aging process but also the possible occurrence of early changes have to be taken into consideration.