This study investigates nitrogen excretion and urea metabolism during early life stages of two species of teleost fish. Although activities of urea cycle enzymes and urea excretion rates were low in adults of the Atlantic cod Gadus morhua L., embryonic and larval cod had much higher enzyme activities and urea excretion rates. Surprisingly, two populations of cod embryos (Newfoundland and New Brunswick) had distinct patterns of urea metabolism. The tissue distributions of urea cycle enzymes were measured in newly-hatched rainbow trout Oncorhynchus mykiss. The key urea cycle enzyme, carbamoyl phosphate synthetase (CPSase III), was localized to the embryonic body (presumably skeletal muscle tissue), not the liver. My study demonstrates that urea is quantitatively a much more important nitrogen end-product in early life stages than in adults of Atlantic cod. Tissue distribution of urea cycle enzymes in rainbow trout contrasts markedly with the typical hepatic urea cycle found in elasmobranch fishes and higher vertebrates.