Hot-film constant temperature probes, evaluated in regimes of pipe-flow, have been used to explore velocity distributions across diameters at various sites in the aorta of dogs. Velocity profiles at all levels were blunt, with boundary-layers less than 2 mm thick; marked skews occurred in the profiles at certain sites in the proximal aorta. The major factors influencing flow in these regions are discussed, and possible causative factors identified. Fully-developed turbulence occurred in only one case, which is described.