Abstract
1. 1. NPN concentration in liver showed higher values at the 9th-48th hr in T. obscurus and at the 12th hr in T. brevispinis after the direct transfer from fresh water to sea-water as compared to their initial levels in fresh water specimens. 2. 2. In muscle of both species, NPN concentrations significantly increased within 24 hr of the transfer to sea-water and decreased to their initial levels in fresh water at the 72nd hr after the transfer. 3. 3. On the other hand, no remarkable changes were observed in NPN concentrations in plasma of both species. 4. 4. These results suggest that: (1) the non-protein nitrogenous compounds play an active role in osmotic regulation of liver and muscular tissues in these fishes and (2) there are the possibilities for release or uptake, and shift of NPN from one tissue to another.
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