The aim of this trial was to evaluate the effect of dietary nucleic acids on ureagenesis and nitrogen balance in gilthead sea bream juveniles. For that purpose, 5 isonitrogenous (7.25% N) diets were formulated based on fish meal as protein source (FM-control diet) and to partially replace the fish meal nitrogen with two levels of nucleic acids (diets RNA1 and RNA2) or brewers yeast (diets BY1 and BY2). Each diet was fed for 10 weeks, to apparent visual satiety, to triplicate groups of 50 fish each with an average body weight of 12.7 g. Inclusion of nucleic acid N either as brewers yeast or RNA led to significant improvement of feed intake and growth, except for fish fed diet RNA2. There were no differences in final weight between groups fed the yeast- and the RNA-containing diets. Although there was an improvement of growth rate of fish fed the RNA or yeast containing diets, dietary nucleic acid supplementation did not have a clear N sparing effect. At the end of the trial there were no significant differences in whole body composition among groups, except for the lipid content which was significantly higher in fish fed the Y2 diet. Hepatic glutamate dehydrogenase activity levels did not differ among groups. Ornithine carbamyl transferase activities were very low in all groups, but appear to be induced by high levels of dietary RNA. Fish fed the yeast diets exhibited the highest arginase activity of all groups. No increase of uricase activities was observed in fish fed the RNA supplemented diets. Ammonia and urea excretion of fish fed the RNA-including diets was higher or significantly higher than in the other groups. Plasma urea levels were higher in fish fed the RNA diets than in the other groups confirming excretion data in that dietary nucleic acid influenced urea excretion through uricolysis. In conclusion, substitution of N supplied by fish meal by that supplied by brewers yeast had a positive effect on growth performance of sea bream, while dietary nucleic acid supplementation did not have a clear N sparing effect. At a dietary level of 11.5% of RNA extract, there was no improvement of fish performance compared to the control, possibly due to an insufficient capacity of fish to degrade high levels of nucleic acid. Data on excretion and plasma metabolites strongly suggest that purinolysis contributes also to ureogenesis in gilthead sea bream. (C) 2005 Elsevier B.V. All rights reserved.