Freshwater-and seawater-adapted Oreochromis mossambicus were submitted to 5 different treatments (urophysectomy, sham operation, urophyseal extract injection, saline injection and control) and transferred to brackish water. In freshwater – adapted fish transfarred to brackish water, urophysectomy increases plasma Na+, K+, Ca+ and osmotic concentrations. In seawater-adapted fish transferred to brackish water, plasma Na+ and K+ concentration were lower in control than other treatment group. The utophysis seems to act only in the “fine tuning” of osmoregulatory processes. Adaptations to changes in the salinity of the medium probably are regulated mainly by prolactin and cortisol
