ABSTRACT: Introduction: Sodium hypochlorite and ultrasonic activation have a synergistic and improving effect on canal disinfection. Some authors found irrigant decrease after activation with ultrasonic, while others described an increased concentration in later stages. The aim of this study was to determine if activation of sodium hypochlorite by passive ultrasonic irrigation reduces its concentration compared to a technique without activation. Materials and methods: A ex-vivo descriptive study was conducted with teeth, randomized into two groups: 10 controls and 20 experimental. The hypochlorite of groups undergoing endodontic treatment was collected, and the post-irrigation residual with saline was gathered. The activation by ultrasonic was performed in stage 4.5 with Ultrasonic Scaler NSK®, three cycles of 20 seconds each per tooth. Irrigant concentration was measured by spectrophotometry. Results. In the first 4 stages, there were no concentration differences between groups. Stage 4.5 demonstrated a significant difference between the treated and control group. At saline irrigation stages, there was only a significant difference in stage E5. When activation was performed, the sodium hypochlorite curve maintained concentration values close to 5% in more stages in comparison to the control group. Conclusions: Passive ultrasonic activation demonstrated higher significant concentration of sodium hypochlorite, compared to a technique without activation.