Although the blood pressure lowering effects of desflurane and isoflurane were similar at equi-MAC, we noted a different pattern of response during intervals of rapidly increasing the inspired concentration of desflurane, when substantial increases in SNA, HR, and MAP occurred. Because of the lower potency of desflurane compared to isoflurane, higher concentrations of desflurane are necessary to establish an adequate surgical plane of anesthesia. Although clinically relevant concentrations of isoflurane did not trigger sympathetic activation, isoflurane triggered responses at an inspired concentration (approximately 5%) nearly equal to that of desflurane. The present research demonstrates that the initial exposure to desflurane in clinically relevant concentrations following anesthetic induction and the deepening of anesthesia with higher concentrations of desflurane can be profoundly sympatho-excitatory. Considerable caution should be taken when administering desflurane to patients who may be placed at risk by these responses.