The purpose of this study was to characterize the activity of essential oils from Nepeta Cataria (EON) at concentrations of 125 µí µí°¿.L-1 , 150 µí µí°¿.L-1 , 175 µí µí°¿.L-1 and 200 µí µí°¿.L-1 on the behavior of loss of the posture reflex and recovery of the posture reflex and electrocardiographic activity and recording of the opercular beat of Colossoma macropomum during immersion bathing for a period of 5 minutes, in order to obtain a window for safe use during anesthesia. The fish (23.38 ± 3.5g) were assigned to the following experiments: Experiment 1 (latency to loss and recovery of the posture reflex): a) 125 µí µí°¿.L-1 b)150 µí µí°¿.L-1 , c) 175 µí µí°¿.L-1 and d) 200 µí µí°¿.L-1 (n=9) per group. Experiment 2 (electrocardiographic and heartbeat recordings): a) Control group; b) Vehicle control group (2 ml of alcohol per liter of water), c) 125 µí µí°¿.L-1 d) 150 µí µí°¿.L-1 , e) 175 µí µí°¿.L-1 and f) 200 µí µí°¿ .L-1 (n=9), per group. All the concentrations used showed efficacy in inducing loss of the posture reflex and reversibility with recovery of the posture reflex, but the electrocardiographic recordings indicated morphographic changes such as bradycardia during induction and p-wave apiculation during recovery at the highest concentrations tested. In this way, we suggest a safe use window for short-term anesthesia with EON in the concentration range of 125 µí µí°¿.L-1 to 150 µí µí°¿.L-1 for juvenile Colossoma macropomum.