Ammonia exists in water in equilibrium between the ionized (NH4+) and ionized (NH3) forms, and the toxicity to fish is mainly from the latter. The relative concentration of NH3 in water is strongly influenced by several environmental factors. Generally, the proportion of NH3 increases with increased pH value and temperature, whereas it decreased with increased salinity. The aim of this study is to investigate the effect of short-term (24 hours) exposure of a sublethal total ammonia-N concentration (30 mg/l) on blood components in freshwater and seawater tilapia, Oreochromis mossambicus. The concentration of NH3, obtained by calculation, in fresh and seawater is 1.58 and 3.73 mg/l, respectively. However, ammonia caused same changes of hematological parameters in freshwater and seawater tilapia. The all values of hematocrit, plasma chloride, and total plasma protein were not significantly different among the initial, control and ammonia groups. Serum glucose was the only parameters that showed a significant difference among the threes groups. The tilapia exposed in ammonia solution had a higher value than did the ones in initial and control groups.