Seawater-adapted sea bass (Lates calcarifer) subadults and fingerlings were abruptly transferred to fresh water and freshwater-adapted subadults and fingerlings to sea water. To follow the responses of plasma osmolality and chloride concentration, blood samples were taken before and at intervals after transfer. Changes in plasma osmolality and chloride showed a pattern of initial crisis when osmolality and chloride levels deviated significantly from pre-transfer levels. The initial crisis was followed by a regulatory phase when these values returned to almost pre-transfer levels. Subadults transferred to fresh water had a regulatory phase (or the ability to return plasma osmolality and chloride to near normal levels) starting at Day 4 after transfer while fingerlings transferred to fresh water had a much earlier regulatory phase, starting at Day 1. Subadults transferred to sea water had stable plasma osmolality and chloride levels at Day 2 while fingerlings transferred to sea water regulatory phase commencing at Day 1.