Background: ammonia (NH3) is the main excretion product from protein catabolism in fish, eliminated primarily through the gills. The proportion excreted by each species depends on factors such as protein quality, energy level and diet balance, body size of the animals, and environmental factors such as water temperature and pH. Objective: to determine the effect of dietary protein level (D1 = 250 g/kg, D2 = 300 g/kg, D3 = 350 g/kg) and body weight (P1 = 45 g, P2 = 250 g, P3 = 520 g) on ammonia excretion (AE) in white cachama (Piaractus brachypomus). Methods: basal AE level was determined by measuring water ammonia concentration every 2 h for 26 h after a 48 h fasting period. The AE in response to CP levels was determined for each fish size by measuring ammonia every 2 h for 26 h, after feeding them to satiety with the experimental diets. Results: basal AE was 177.2, 128.7, and 79.2 mg N-NH4+/day/kg live weight (LW) for P1, P2, and P3, respectively. The differences between treatments were significant (p