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The effects of acute and chronic administration of dextroamphetamine (DA) on halothane MAC in dogs were evaluated. Acute intravenous administration of DA, 0.1, 0.5, and 1 mg/kg, during halo thane anesthesia was associated with increases of MAC to 19 ±8, 67 ± 11, and 96 ± 15 per cent above control values. Blood pressure increased 40, 112, and 109 per cent, respectively, at the three dose levels, and in 12 of 15 acute-administration trials the dogs developed cardiac arrhythmias. Body temperature increased 0.4 to 2.1 C following acute administration of DA. The large change in MAC produced by interaction of DA with halothane could be decreased by respiratory alkalosis. In contrast, chronically-treated dogs receiving 5 mg/kg/day of DA intramuscularly for seven days had decreases in MAC of 21 ± 3 per cent from control values. These data support the hypothesis that catecholamines that act on the central nervous system may alter anesthetic requirements.