The effect of Althesin on local blood flow in the brain was studied using intra-carotid Xenon 133 in 18 subjects: 6 apparently normal, 6 with diffuse cerebro-vascular pathology, and 6 cases of ischaemic cerebral accidents identified by scintigraphy, tomodensitometry and angiography. Blood flow decreased under Althesin in the first 2 groups, although to a lesser extent in the second group, with a concomitant decrease in oxygen metabolism. In the third group a "paradoxical" increase in blood flow was observed in infarcts, and a decrease proportional to the rest of the brain in limited ischaemia which had subsequently regressed. The "Althesin test" proposed by Rasmussen (1975) thus reveals the existence of a disconnection between blood flow and metabolism within cerebral infarcts, and provides a possible means of differentiating the latter from reversible ischaemias.