Bovine aortic endothelial cells (BAEC), detached with collagenase, have been studied morphologically after administration, in vitro, of human lipoprotein fractions (200 micrograms/ml of LDL, 200 micrograms/ml of HDL), 24 h before the confluence. One day after the treated cells and the control one were detached with trypsine and prepared for freeze-etching examination. The plasma-membrane has revealed, in the lipoprotein treated cells (HDL or LDL) an increased average value of the number of "pits" present upon a surface unit (P less than 0,05). Moreover, in the HDL-treated cells, the "pits", sometimes, have appeared with a different morphological evidence, similar to the aspects of exocytosis as seen e.g. in platelets or in Langerhans isle cells.