Carotid blood flow in rats was measured by implanted transit-time ultrasonic flowprobes throughout hyperbaric experiments conducted up to 70 bar (7 MPa) with a helium-oxygen hyperoxic (PO2 = 400 mbar) mixture. Before the hyperbaric experiment, an intracerebroventricular (ICV) injection of phosphate saline-buffered solution (PBS) or adenosine deaminase (ADA, 100 U.ml-1) in PBS was performed. Throughout the hyperbaric experiment carotid blood flow increased with ambiant pressure in PBS-treated rats. Conversely, the increase in carotid blood flow was attenuated by ADA treatment. These results suggest that the increase in carotid blood flow at high ambiant pressure could result from an increase of adenosine concentration in the rat brain.