Tissue cultured bovine aortic endothelial cells were subjected to flow in an in vitro circulatory loop which was designed to simulate the flow and pressure conditions in the aorta. The cells were cultured to confluence under stationary conditions on a tube consisting of microfabric backed with polyurethane. The tube with the cultured cells was then added to the flow loop which circulated complete tissue culture medium in a pulsatile mode. Light microscopy, and scanning and transmission electron microscopy showed that the cells not only remained adherent for 2 week periods under flow conditions, but also underwent hypertrophy and proliferation in response to the flow regimen.