Increases in branchial Na(+)/K(+) ATPase activity during seawater adaptation of euryhaline fish species, have been well documented. During the parr-smolt transformation of salmonids this activity increases two to five fold and is used as an indicator of the transformation. In order to improve the understanding of differences in enzyme activity found between Atlantic salmonSalmo salar parr and smolt fish, we investigated the gene expression of the Na(+)/K(+) ATPase α-subunit(s) in gill tissue. Gill mRNAs were analyzed and quantified at distinct time points using Northern and Dot blot techniques. We amplified by PCR, a conserved region of the cDNA encoding the Na(+)/K(+) ATPase α-subunit of the rainbow troutOncorhynchus mykiss. The PCR products (670 bp) were cloned and all independent clones showed a sequence corresponding to the α subunit of the Na(+)/K(+) ATPase. The fragments obtained appeared as a heterogenous population of three sequences showing, when compared between each other, 86 to 93% identity. This suggests that different allelic forms of the α-subunit are expressed in gill tissue. Hybridization studies performed with these PCR probes revealed two mRNA species, a major 3.7 kb transcript and a minor transcript of 1.8 kb. Enhanced 3.7 kb transcript levels are concurrent with elevated enzyme activity in smolts during the March and April parrsmolt transformation of Atlantic salmon. Interestingly, our study disclosed that smolt fish only displayed a two-fold increase in transcript levels when compared to parr whereas enzyme activity showed a 4 to 5 fold increase. This suggests that the increase in the 3.7 kb mRNA content of gill tissue is probably not the only mediator leading to the rise in enzyme activity during parr-smolt transformation.