With the current developmentandincreasing number of nanotechnology products,nanotechnology industry has become an importantpillarin the global economy. Aquatic environments that are very vulnerable, act aslocations for deposition and accumulation of the nanoparticles. Among the wide range of the nanoparticles, silver nanoparticles are widely used for their antimicrobial properties. The aim of this study was to assess the possible effects of silver nanoparticles on some physiological characteristics of rainbow catfish, Pangasianodon hypophthalmus . Eighty-five fish (12 ± 0.26 g, 10.7 ± 0.46 cm) were randomly allocated to 5 glass tanks (100 L). The experiment consisted of five treatments including: 1 ?g/L silver nanoparticles (1Ag-), 1 ?g/L silver nitrate (1AgNO3), 20 ?g/L silver nanoparticles (20Ag-), 20 ?g/L silver nitrate (20AgNO3) and control and lasted for 10 days. Fish liver sampling was performed at days1, 5 and 10 (end of experiment) for Metallothionein (MT) gene expression analysis. Gill tissues were fixed in 10% formalin for histopathological studies at the last day of expousure. After 1 and 10 days of exposure, the blood was sampled and common hematological parameters were evaluated. A 62 bp cDNA motif of Mt was sequenced for the first time in Rainbow catfish. Relative gene expression of MT in the liver was measured using Real-time PCR method with 18s as an internal control and was significantly higher in the 20Ag- treatment comparedto the control in all sampling times (p 0.05). Hyperplasia, shortening and looping of secondary lamellae were the main gill histopathological signs of rainbow catfish exposed to water born Ag and AgNO 3 . The mean red blood cell counts (RBC), hematocrite and hemoglobin was increased in 20Ag on the first day (P 0.05); while there was no difference amongst the treatments at the last day of exposure (P 0.05). In both first and last days, there were no significant differences in secondary hematological indices (P 0.05). Based on the results of this study, the toxicological effects of Ag, especially at high concentration, was obvious on the catfish, so it is necessary to consider these side effects of Ag as an antimicrobial componentin aquaculture industry. Key words: silver nanoparticl, hematology parameter, gill, Metallothionein, Rainbow catfish, Pangasianodon hypophthalmus .
