&NA;The effects on cerebral metabolism and the electroencephalogram (EEG) of combining hypocapnia with hypotension have been only incompletely examined. The present study examined the possibility that hypocapnia may worsen the cerebral metabolic and EEG disturbances caused by hypotension. Cerebral metabolism and the EEG were studied at three levels of hypotension during hypocapnia (PaCO2= 20 mm Hg) in dogs under light halothane anesthesia. A sequential decrease of the mean arterial pressure (MAP) to 60, 50, and 40 mm Hg (30 minutes at each level) was achieved with sodium nitroprusside (SNP) (n = 12) or trimethaphan (TMP) (n = 12). With SNP‐induced hypotension plus hypocapnia, the power of the &agr; and &bgr;2spectra of the EEG decreased at MAP ≤ 60 mm Hg. Cerebral metabolic values were unchanged at a MAP of 60 or 50 mm Hg. Brain tissue phosphocreatine and the cerebral energy charge decreased, and the lactate/pyruvate ratio increased at a MAP of 40 mm Hg. With TMP‐induced hypotension plus hypocapnia, power decreased in the &agr; and &bgr;2spectra of the EEG at MAP ≤ 60 mm Hg. Cerebral metabolic values were unchanged at a MAP of 60 mm Hg. At MAP ≤ 50 mm Hg, power in the &bgr;1spectrum, brain tissue phosphocreatine, and the cerebral energy charge all decreased. At a MAP of 40 mm Hg, the cerebral glucose value decreased and the lactate/pyruvate ratio increased. The cerebral metabolic and EEG consequences of hypotension plus hypocapnia observed here were similar to those previously reported to occur with SNP‐ or TMP‐induced hypotension at a normal PaCO2. In these studies, hypocapnia did not intensify the cerebral impact of SNP‐ or TMP‐induced hypotension. (Neurosurgery18:36‐44, 1986)