The authors determined the effects of aminophylline on the anesthetic requirements for halothane in rats and dogs. MAC for halothane was determined in rats (n=24) before and after aminophylline, 100 mg · kg−1ip, or an equal volume of saline. Because changes in central noradrenergic neurotransmission have been linked to drug-induced changes in the depth of the anesthetic state, we investigated the effect of aminophylline on the turnover of norepinephrine in discrete brain regions of halothane-anesthetized rats. To facilitate testing at steady-state aminophylline conditions and to permit frequent blood sampling, halothane MAC was determined in dogs (n=7) before and after a therapeutic level of aminophylline (15 ± 2 µg · ml−1) was obtained. Neither in the rats (1.0 vs. 1.0%) nor in the dogs (1.04 ± 0.14 vs. 1.01 ± 0.14%) was halothane MAC affected by aminophylline treatment. Commensurate with the lack of change of anesthetic depth, aminophylline treatment did not affect noradrenergic neurotransmission in the brain of halothane-anesthetized rats. Furthermore, the anticipated increase in circulating catecholamines following aminophylline treatment in dogs did not materialize. The authors conclude that halothane anesthetic requirements are not altered by aminophylline treatment, possibly because of the attenuation of the putative sympathomimetic effects of aminophylline by halothane.