The osmoregulatory capabilities of 6-month-old juvenile obscure puffer Takifugu obscurus, transferred directly from fresh water to different salinities (0‰, freshwater control; 10‰; 20‰ and 30‰), were studied over an 8-day period. After transfer, plasma osmolality of the fish at 30‰ was significantly higher than those at all other salinities throughout the experiment. The Naþ/Kþ ATPase activity in the gills of the fish treated with various sa- linities increased significantly, peaking at 48 h, then decreased gradually to the control level at 192 h. Similar fluctuation trends of the Naþ/Kþ ATPase activity were observed in the kidneys. Modified Gaussian model provided accurate fits for the time-course changes in the Naþ/Kþ ATPase activities after abrupt salinity challenge. The results demonstrated that obscure puffer has strong capacity to tolerate abrupt salinity changes and can osmoregulate well over a wide range of salinities even in juvenile stage.