The study was done in  nutrition and live food in aquaculture research station was located in Bandar Anzali Ghaziyan. Juveniles weighted average 0.5, 1, 2.5 gr were randomly selected in three groups Caspian Sea with a salinity of 11 ppt, water 7 ppt and freshwater (with three replicates per group) were included.

To study the microstructure of gill for each treatment , tissue samples by classical histological methods and stained with hematoxylin - eosin slides were prepared. The frequency and location of the enzyme Na ⁺, K ⁺ - ATPase and chloride cells with immunohistochemical localization was performed. Studies micrometric  gill chloride cells by software Image tool (version 2.0) was performed. Data analyzed by one-way ANOVA (Oneway ANOVA) with Tukey's test was performed. Results of gills chloride cells  number per square millimeter of the surface  gills in Rutilus frisii kutum fingerlings showed that number of cells  in 0.5, 1 and 2.5 gr juveniles were 9000, 9000 - 10000 and 10070- 12060 cells respectively. .Overall, the results of measuring ions and osmotic pressure on the tenth day of treatment, the osmotic potential juveniles 2.5 gr in Caspian sea water and   all groups except the juveniles 0.5 gr  in water of 7 ppt confirmed.

Key words: Rutilus frisii kutum, osmolarity, suitable size, release, Caspian sea, Iran