Hypoosmoregulatory activity of two salmonid species (one smoltifying and one non-smoltifying) was studied by direct transfer of these fish into sea water. Two plasma parameters were followed during the experiments: osmotic pressure and sodium level. Using an homologous radioimmunoassay for salmon prolactin, the plasma level of this hormone was also measured. Seawater adaptation of rainbow trout appeared to be difficult, as shown by the high increase of osmotic pressure and sodium level. However, after 2 weeks the fish were able to regulate their hydromineral balance. Prolactin level dropped within 24 h after transfer and remained significantly lower than in the freshwater fish during the experiment. In contrast, Atlantic salmon smolts transferred to sea water were able to control their hydromineral balance within a few days after transfer, as shown by plasma osmotic pressure and sodium level. Moreover, prolactin level was not significantly different between freshwater and seawater fish from day 4 to the end of the experiment. Thus, by measuring plasma prolactin level, it was possible to confirm the two patterns of seawater adaptation presented by Atlantic salmon smolt and rainbow trout. These results are discussed in terms of a possible role of prolactin in salmonid osmoregulation.