Rainbow trout aquaculture is growing rapidly, as is the body of research that supports this industry. However, there remains a need for better understanding the fundamentals of the smoltification process. This thesis examines the effect of two different brackish water treatments on smoltification in addition to standard smoltification in freshwater of rainbow trout (Oncorhynchus mykiss). The project includes two experiments: 1. Smolt development and performance during salinity treatment 2. Seawater response and performance after smoltification during salinity treatment. Smolt development and seawater response was characterized using gill mRNA transcription (nkaα1a, nkaα1b and nkcc1a), gill Na+ /K+ - ATPase activity and plasma ion concentration (Cl- and Na+ ). Moreover, growth of the different salinity treatments and after seawater transfer was recorded. Rainbow trout were exposed to a water salinity of approximately 1 ‰ (freshwater control group), 4 ‰ and 15 ‰ in the course of smoltification. Salinity induced an earlier smolt development by activating a rapid change in the mRNA transcription of the α-isoforms nkaα1a (freshwater isoform) and nkaα1b (seawater isoform), which, in turn, induced an earlier peak of the Na+ /K+ -ATPase (NKA) activity. Salinity had no effect on growth during smoltification. Rainbow trout transferred during the preparatory smoltification phase differed from the rainbow trout transferred after the completion of smoltification. This early transfer induced an upregulated mRNA transcription of the seawater α-isoform nkaα1b and the co-transporter nkcc1a after seawater entry. It also induced an increase in the specific growth rate (SGR) of weight. No major differences were observed among treatment groups, suggesting that timing of seawater transfer is more important than salinity rearing conditions.