This study evaluated anesthetic efficacy and possible effects of the essential oils (EOs) of Cunila galioides (EOC) and Origanum majorana (EOO) on ventilatory rate (VR) and ionoregulation in Rhamdia quelen. In the anesthesia assessments, 50, 100, 200 and 300 μL L -1 EOC and 50, 100, 200, 300, 400 and 500 μL L -1 EOO were tested, and time for induction to sedation and anesthesia stages, as well as recovery, were taken. A second trial employed lower concentrations of both EOs, 10, 25, 50 and 100 μL L . There was no significant difference between control and EO-treated groups regarding VR, but all fish subjected to 100 µL L -1 EOC died within 2 h of exposure. Overall, ionic loss declined in the presence of the EOs. The EOC at 200 -300 μL L -1 and EOO at 400 -500 μL L -1 present the potential to promote fast anesthesia in R. quelen.