The effects of diazepam on cerebral blood How (CBF) and cerebral oxygen uptake(CMRo2) was studied using a 133xenon modification of the Kety— Schmidt (1948) technique in paralyzed, artificially ventilated nits with and without simultaneous administration of 70 per cent nitrous oxide. Diazepam was given iv in doses that induced light to heavy sedation or general anesthesia. When given with 70 per cent nitrous oxide, diazepam in sedative and anesthetic doses lowered CBF and CMRo2to about GO per cent of control. In the absence of nitrous oxide all doses of diazepam caused moderate (20–30 percent) decreases in CBF, but CMRo2remained unchanged or was only slightly lowered. It is concluded that diazepam interacts with nitrous oxide to produce a reduction in CMRo2similar to that seen in barbiturate anesthesia, but that alone the drug produces sedation and anesthesia without a comparable decrease in CMRo2.