In rhesus monkeys, data obtained by contrast radiography and hemodynamic and electroencephalographic studies indicate that carotid blood flow is reversible. Results showed that even small amounts of local anesthetic agents when injected inadvertently into a branch of the external carotid artery, may enter the cerebral circulation, most likely through a retrograde flow into the common and then internal carotid arteries. Some toxic neurologic manifestations possibly may be explained by this mechanism.