Abstract: The toxicity (LC50) of total and un-ionized ammonia was tested on different sizes (5, 10, 15 g) of common carp Cyprinus carpio maintained at three different pH levels within the range of 6.5-8.5. Within the same pH, the toxic effect of ammonia was independent of fish size. In addition, the toxicity of un-ionized ammonia increased at lower pH. However, total ammonia exerts some measure of toxicity and/or increased H+ concentration increases the toxicity of un-ionized ammonia. Common carp fingerlings (15 g) were exposed to the 96 h LC50 of un-ionized ammonia (0.93 mg NH3-N/L) at pH 7.5. Changes in hemoglobin (Hb), hematocrit (Ht), sodium (Na+), potassium (K+), glucose, aspartate aminotransferase (AST), alanine aminotransferase (ALT) and lactate dehydrogenase (LDH) were recorded. Blood Hb decreased after 6 and 96 h of exposure while Ht decreased after 3 h of exposure. Serum Na+ and K+ increased during the experimental period (96 h). Glucose concentration increased initially and then returned to less than the control value after 96 h of exposure. There was a significant decrease in AST, ALT and LDH activities after 6 h of exposure, after which enzyme activity increased until the end of the experimental period.