Abstract The effect of hormone treatment on the abundance of Na +-K +-ATPase α- and β-subunit mRNA in Sparus sarba branchial tissue was investigated. Groups of seawater (33‰) and hypo-osmotic (6‰) acclimated fish were injected daily, with either saline, cortisol, recombinant bream growth hormone (rbGH) or ovine prolactin (oPRL). Total RNA from branchial tissue was analyzed by Northern blotting using PCR amplified Na +-K +-ATPase α- and β-subunit cDNA clones. Na +-K +-ATPase α- and β- subunit transcripts of 3.3kb and 2.4kb respectively, were detected and their abundance, after hormone treatment was assessed using RNA dot blots. The abundance of subunit mRNAs increased 1.4–1.9 fold, relative to controls, after cortisol treatment. The α:β mRNA ratio also increased in cortisol treated seawater acclimated fish. Growth hormone treatment did not cause any significant changes in Na +-K +-ATPase subunit mRNA, whereas prolactin significantly reduced α-subunit mRNA levels by approximately 0.5 fold in both seawater and hypo-osmotic conditions. The data from this study add further support to the generally accepted roles that cortisol and prolactin have in the modulation of Na +-K +-ATPase activity. It can be concluded from this study that S. sarba branchial Na +-K +-ATPase subunit expression is multihormonally regulated.