Cerebral blood flow (CBF) is higher during drug-induced hypotension than during hypotension resulting from hemorrhage and, among the hypotensive drugs, CBF may be higher during nitroprusside (SNP) than during trimethaphan (TMP) administration. Increased perfusion of the brain does not guarantee better tissue oxygen supply, since perfusion of the capillary microcirculation may be inhomogeneous. In such a situation, minute areas of ischemia may exist while CBF values are normal. In this study, regional cerebral blood flow (rCBF) was measured in circles of parietal cortex approximately 1.5 cm in diameter in the cat. In addition, oxygen tension values (PB02) were measured in minute areas of cortical surface by use of 15-μm platinum oxygen electrodes. Control measurements were made during light anesthesia; blood pressure (BP) was then decreased to 30–35 torr by hemorrhage, TMP, or SNP. In the animals rendered hypotensive with TMP or SNP, a beta blocker, practolol, was also administered. It was found that rCBF was higher with SNP (68 ml/100 g/min) than with TMP (45 ml/100 g/min). PB02 values showed a marked hypoxic shift during hemorrhagic hypotension, but no shift from control during SNP-induced hypotension. The PB02 pattern with TMP was intermediate between the SNP and hemorrhage patterns. PB02 were less than 10 torr in 25 per cent of cortical areas examined during hemorrhage, 12 and 7 per cent of areas examined during TMP-induced hypotension (two studies) and only 1 per cent of areas examined during SNP-induced hypotension. These results agree with those of others in showing higher rCBF during SNP-induced than during TMP-induced hypotension. The PB02 values showed that the well-maintained cerebral perfusion during SNP administration was associated with homogeneous perfusion of the microcirculation. Therefore, brain tissue oxygen availability is greater during SNP-induced than during either TMP-induced or hemorrhagic hypotension.