The aortic endothelium is a naturally occurring monolayer which allows comparison between wound healing in the intact animal and similar phenomena studied with monolayers in tissue culture. We used the balloon catheter technique to abrade a defined portion of the lining of the aorta in rats. Proliferation and migration across the line of injury were quantitatively determined using tritiated thymidine autoradiography, incidenct light microscopy, and scanning electron microscopy. The findings in vivo differ from those in vitro in three ways:(1) The response in vivo is not limited to the line of injury. Cells up to 100 cells behind this line are stimulated to enter S phase and undergo a characteristic change in shape not seen in the tissue culture systems.(2) This same" regeneration zone" escapes normal controls on cell density, producing three times as many cells per unit area as in unstimulated endothelium.(3) Cell migration, but not proliferation, occurs preferentially along the axis of the vessel, equally well against and with blood flow. These findings differ from results of tissue culture studies and suggest that the" diffusion barrier" mechanism proposed for control of monolyaer regeneration in vitro does not explain contact inhibition of endothelium in vivo and that regeneration of endothelium is controlled either by factors released from the site of injury or by cell to cell interactions.