Transfer experiments to sea water were conducted, using rainbow trout and goldfish, to investigate the early changes in plasma osmolality and sodium and chloride concentrations for 60 minutes after transfer. The results obtained are as follows. In rainbow trout, plasma osmolality increased and attained the highest lovel 15 minutes after immersion in sea water. Thereafter plasma osmolality gradually approached a little lower level. During this experimental period, changes in plasma sodium and chloride concentrations were not conspicuous and all fish survived. On the contrary, in goldfish direct transfer to sea water induced the death of all experimental fish. Average survival time was about 33 minutes. For this short period, plasma osmolality and also sodium and chloride concentrations increased remarkably and finally attained the levels of survival limitation.