Abstract In Brazil, pre-slaughter immersion in ice is used in most fish slaughterhouses. We aimed to study the parameters for electrical stunning in pacu (Piaractus mesopotamicus) as a way to reduce animal suffering during slaughter and the stunning effects on meat pH and rigor mortis. We evaluated 70 fish, divided into two groups. In group G205, fish were stunned with 205 Volts (V), 50 Hz, duty cycle of 70% and 1.3 Amp (A) applied for 45 s; in group G400, fish were stunned with 400 V, 30 Hz, duty cycle of 30% and 0.9 A, for 30 s. Electrical stunning was applied using a backpack electrofisher (Smith Root LR-24) in a plastic box with 80 L of fresh water. We determined the unconsciousness time through behavioral evaluation. Immediately after checking the return of consciousness, all fish were placed in a plastic box with 35 L of fresh water and 200 mg L−1 of benzocaine before bleeding. After slaughter, the first 10 fish from each group were identified and placed in refrigerated plastic boxes with ice for measurements of pH in the epaxial muscle as well as the rigor mortis index (RMI) at 0, 3, 6, 24, 72, 120 and 168 h after bleeding. All fish were effectively stunned. When the current was applied, we observed that all fish lost behavioral responses and reflexes instantly: fish stopped swimming, lost equilibrium, did not respond to painful stimuli, ceased opercular movements and did not show vestibulo-ocular reflex. The unconsciousness time was significantly higher in the G205 treatment, equal to 61.7 ± 13.4 s (P .05). All fish reached maximum rigor mortis (RMI = 100%) within 6 h post mortem. There was no external damage, vertebral fracture, hemorrhages or blood spotting in the meat. Electrical stunning in pacu seems to be feasible through the G205 parameters, due to the unconsciousness period longer than 1 min, which may allow minimum time period to proceed bleeding without suffering. More research is needed to provide best strategies to the fish industry, so that fish regulations regarding humane slaughter may be implemented.