Measurements of the rate of nitrogenous excretion were carried out in two batches of young Black Sea turbot (small 42 g, large 72 g) at 12 +/- 1 degrees C under natural light conditions (10 h light:14 In dark and 13 h light: 11 h dark, respectively). The ammonia nitrogen excretion rates of fish starved for 48 hours were 0.20 +/- 0.05 mg-N for the small fish and 0.18 +/- 0.09 mg-N/100g fish/h for the large. Fish were then fed a pellet diet containing 8.3% nitrogen at average rations of 0.67% and 0.59% of the body weight, respectively, for four days. On the fourth day, ammonia nitrogen excretion rates were evaluated. In both batches, the rates were 2-3 times higher immediately after feeding than in the starved fish, reaching a peak 3-6 hours after feeding and declining afterwards. For the small and large fish, respectively, 21% and 20% of the consumed nitrogen was excreted as ammonia nitrogen, 6% and 7% as urea nitrogen, and 8% and 4% as feces nitrogen within 24 hours after feeding.