Abstract: Objective To investigate the effect of different fresh gas flows (FGFs) on the pharmacodynamics of isoflurane during anesthesia induction. Methods Sixty female ASA class Ⅰ or Ⅱ patients (aged from 18 to 49 years) scheduled for gynecologic laparoscopic surgery were randomly divided into groups Ⅰ, Ⅱ, and Ⅲ (n=20). The FGFs for group Ⅰ, Ⅱ, and Ⅲ was 1, 2 and 3 L/min, respectively, and each group was further divided into two equal subgroups according to the setting concentrations of isoflurane vaporizer (Co), which was 1% in groupsⅠ1, Ⅱ1, and Ⅲ1 and 2% in groupsⅠ2, Ⅱ2, and Ⅲ2. Isoflurane at different setting concentration was administered under different FGF in the patients after tracheal intubation following anesthesia induction. The systolic blood pressure (SBP), diastolic blood pressure (DBP), main arterial blood pressure (MAP), heart rate (HR) and bispectral index (BIS) were recorded before anesthesia induction and every 3 min after tracheal intubation. Patients given ephedrine and atropine were also recorded. The patients’ consciousness during anesthesia were followed up and recorded. The inspiratory concentration (CIiso) and expiratory concentration (CEiso) of isoflurane in the airway were monitored and recorded every 3 min. The observation after intubation lasted for 18 min, during which stimulation of the patients was avoided, and the operation began after the observation. Results There was a close correlation between BIS and CIiso and between BIS and CEiso (r=-0.904 and -0.893, respectively). The incidence of hypotension was significantly different between groups Ⅲ andⅠ (P<0.01), and between the subgroups in groups Ⅱ and Ⅰ (P<0.05). No bradycardia occurred and no consciousness reported awareness during anesthesia. Conclusions Between the completion of tracheal intubation and beginning of the surgery, 1% or 2% Co under a moderate FGF (1-3 L/min) may guarantee the patients’ unconsciousness, but hypotension is less likely under a relatively low flow (1-2 L/min) than a higher flow (3 L/min). Higher FGF and Co result in faster induction of deep anesthesia and higher incidence of hypotension.