Diurnal pattern of excretion of ammonia-N and urea-N through the renal and extra-renal sources were studied in a freshwater air-breathing teleost,Heteropneustes fossilis in its aquatic medium. Ammonia-N was the major nitrogen excretory product, however, the rate of excretion of urea-N was comparatively higher than other freshwater teleosts. Almost all ammonia-N (>99%) and about 75% of urea-N were excreted through the extra-renal sources. The excretion rate of urea-N was about 10 times higher than ammonia-N through the renal and ammonia-N was 7–8 times higher than urea-N through extra-renal sources. Existence of a functional ornithine-urea cycle reported inHeteropneustes fossilis has been suggested to be the reason for this increased rate of urea excretion.
Nitrogen excretion (total ammonia-N + urea-N) during the night was significantly higher than the day. A higher rate of nitrogen metabolism at night has been suggested forHeteropneustes fossilis which has been reported to be nocturnal in habit.