Cerebral circulation and metabolism during Althesin anaesthesia were studied in seven healthy patients. Althesin was given in a single dose of 0.1 ml/kg and thereafter infused at a constant rate of 0.3 ml/kg/h. During Althesin infusion, the cerebral blood flow (CBF), the cerebral metabolic rate for oxygen (CMRo2) were 29 +/- 10 ml/100 g/min and 1.7 +/- 0.4 ml/133 g/min, respectively. These values were significantly different from those obtained in awake subjects in our laboratory (CBF: 46 +/- 7 ml/100 g/min; CMRo2: 3.1 +/- 0.6 ml/100 g/min). During CBF measurement, the mean cerebral perfusion pressure, cerebral vascular resistance (CVR) and arterial carbon dioxide tension (PaCO2) were 89 +/- 16 mm Hg, 3.4 +/- 1.3 mm Hg/ml/100 g/min, and 36 +/- 9 mm Hg, respectively. The relationship between CBF and PaCO2 were studied and it was found that during Althesin anaesthesia reactivity of cerebral vessels to the alteration of PaCO2 was maintained. It is concluded that Althesin caused cerebral metabolic depression which was accompaned by a decrease in CBF and an increase in CVR.