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SYNOPSIS C02 responsiveness of the cerebral circulation has been measured in baboons before, during, and after halothane-induced hypotension. At a systolic blood pressure (BP) of 60 mmHg, CO2 responsiveness was abolished, but was maintained at higher levels of BP. After hypotension, CO2 responsiveness returned to control values. Autoregulation to BP increases induced by intravenous noradrenaline was impaired when cerebral perfusion pressure during the hypotensive period had been below 30-40 mmHg. It is concluded that at levels of halothane-induced hypotension commonly employed clinically, CO2 responsiveness of the cerebral circulation may be absent. The return of CO2 responsiveness in the post-hypotensive phase argues in favour of controlled hyperventilation after neurosurgery which has involved induced hypotension.Harper and Glass (1965) reported loss ofcerebrovascular responsiveness to Paco2 changes during haemorrhagic hypotension, while Haggendal and Johansson (1965) reported that some degree of response was retained in these circumstances. Recent evidence (Fitch et al., 1973) shows that the cerebral circulation responds differently during drug-induced hypotension compared with hypotension produced by haemorrhage.It seemed important, therefore, to establish whether changes in cerebrovascular responsiveness to PaC02 occurred during drug-induced hypotension, since the combination of hypotension and hyperventilation could, by a combined action on cerebral perfusion, be clinically undesirable. Consequently, we decided to investigate cerebrovascular responses to changes in PaCO2 during hypotension induced with halothane at a level of systolic blood pressure commonly employed clinically. METHODSFive baboons weighing between 7.2 and 13.8 kg were premedicated with phencyclidine (0.8-1.0 mg/kg) intramuscularly and anaesthesia was induced and (Accepted 22 October 1975.) 221 maintained with halothane, N20, and 02. After intramuscular injection of suxamethonium (50 mg), the animals were intubated and ventilated by intermittent positive pressure (Starling ventilator

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