We recently showed that a series of tight junction proteins of the claudin family are regulated in the gill of salmon during salinity acclimation. The aim of the present study was to investigate the role of cortisol, growth hormone (GH) and prolactin (PRL) on regulation of expression of these isoforms. Experiments on primary cultures of gill tissue showed that cortisol stimulates claudin 10e, 27a and 30 mRNA levels while no significant effects were observed on claudin 28a and 28b. The associated receptor signalling pathway was examined using glucocorticoid and mineralocorticoid receptor antagonists RU486 and spironolactone, respectively. The observed in vitro responses were blocked by RU486, suggesting the involvement of a glucocorticoid type receptor. Injections of FW salmon with cortisol increased the expression of claudin 10e, 27a, and 30 but did not affect claudin 28a and 28b significantly. While GH had no effect on its own, the combination of GH and cortisol reduced claudin 28b levels. Injection of SW salmon with PRL selectively increased the expression of claudin 28a but had no effect on the other examined isoforms. The data shows that FW- (27a and 30) and SW-induced (10e) claudins are all stimulated by cortisol while the major osmoregulatory hormones GH and PRL had no effect on these salinity sensitive isoforms. This suggests that other hormones and/or osmotic conditions interact with cortisol to determine claudin composition in the gill.