Local blood flow, cerebrovascular autoregulation and CO2 responsiveness in the rabbit hypothalamus.

1. Local blood flow has been measured in the hypothalamus and other areas of the brain in the conscious rabbit, by measuring the rate of clearance of small volumes (2.5-10 mul.) of a saline solution of the inert tracer (133)Xe injected locally.2. Increasing the arterial P(CO2) caused a mean rise in the hypothalamic blood flow of 34%.3. The hypothalamic blood flow remained relatively constant over a mean arterial blood pressure range of 41-140 mm Hg.4. Hypothalamic blood flow was relatively unaffected by anaesthesia with intravenous pentobarbitone; by contrast, blood flow in the cerebral cortex was greatly reduced.5. It is concluded that the technique is a valid and useful one, since CO(2) responsiveness and autoregulation is maintained. Different areas of grey matter have different flow rates, and vary in their responsiveness to barbiturate anaesthesia.