Pinocytotic vesicles are one of several anatomic factors relevant to the permeability for macromolecules of capillary endothelia. We quantified the number and distribution of pinocytotic vesicles present in the endothelium of pulmonary capillaries. Several similar studies have been reported in the past for endothelia of systemic capillaries, but the technical difficulties involved in this difficult application of morphometry never have been discussed. In the present work, an on-line microprocessor was used to assist with the processing of the point-counting data. The average figures from nine rabbit lungs for parameters thought to be relevant to characterize the population of vesicles are: thickness of the endothelium (TAUen), 0.192 fim; number of vesicles per volume unit of cell (Nv—), 131 vesicles//im3; vesicular load of each cell front (N/S)b, 196 vesicles//im2, and (N/S)i, 181 vesicles//im2. The number of vesicles per volume unit (Nvra) is smaller than that which has been described for systemic capillaries.Circ Res 47: 384-391, 1980