Laboratory experiments on juvenile turbot. Scophthalmus maximus (L.), were carried out under controlled temperature (12°C) and feeding regimes in a flow‐through system. Monitoring of total ammonia nitrogen (TAN) and urea nitrogen (Urea‐N) was performed through continuous sampling of the effluent sea water. The effects of ingested nitrogen levels on TAN and Urea‐N daily and hourly excretion rates were studied. Strong relationships were found between ingested nitrogen, and both TAN and Urea‐N excretion for both daily and hourly maximum excretion rates. Turbot showed a low metabolic activity, confirmed by low excretion levels. Daily patterns for TAN and Urea‐N production were different, suggesting specific physiological phenomenon for urea excretion mechanisms. The question of whether turbot are partly ureotelic or ureogenic is underlined.