After occlusion of an artery to the brain, hypercarbia and seizures may produce a paradoxical reduction in cerebral blood flow to the region supplied by collateral vessels. We measured pressure in an occluded branch of the middle cerebral artery and measured regional cerebral blood flow (rCBF) to collateral-dependent cerebrum in dogs (n = 25) to examine hemodynamic mechanisms that account for the reduction in flow. During hypercarbia (arterial PCO2 = 70 +/- 5 mmHg), rCBF to collateral-dependent cerebrum, measured with microspheres and identified using the shadow flow technique, decreased from 95 +/- 6 (mean +/- SE) to 71 +/- 9 ml.100 g-1.min-1 (P < 0.05), while flow to normal brain increased from 105 +/- 9 to 281 +/- 15 ml.100 g-1.min-1 (P < 0.05). Pressure in a branch of the middle cerebral artery decreased during hypercarbia from 50 +/- 6 to 25 +/- 3 mmHg (P < 0.05), concurrent with a significant increase in resistance of collateral vessels. Small vessel resistance was the same in collateral-dependent and normal brain. During bicuculline-induced seizures, with blood pressure maintained at control levels by withdrawal of blood, rCBF decreased in collateral-dependent cerebrum from 128 +/- 16 to 67 +/- 11 ml.100 g-1.min-1 (P < 0.05), and flow to normal brain increased from 169 +/- 14 to 418 +/- 17 ml.100 g-1.min-1 (P < 0.05). Small vessel resistance decreased in both regions, but the decrease was much greater in normal cerebrum. Changes in cerebral artery pressure and resistance of collateral vessels during seizure were similar to those during hypercarbia.(ABSTRACT TRUNCATED AT 250 WORDS)