Turbot juveniles were exposed to four ammonia concentrations [0.17 (L), 0.34 (M), 0.73 (MH) and 0.88 (H) mgl(-1) NH3-N] for different exposure durations (28 days minimum to 84 days). Their physiological status and growth performances were compared to a control group [0.004 (C) mgl(-1) NH3-N]. No growth was observed in the H group, and by day 57, mass increase in the MH group was only 15% of that in group C. During the first month growth in the L group was similar to that in control group while it was lower (33%) in the M group; afterwards the L and M groups had a similar growth (half that of controls). Accumulation of total ammonia nitrogen (TA-N) in plasma was dependent on ambient ammonia concentrations. Plasma urea levels in ammonia-exposed fish were lower, similar or greater than ill controls (depending oil ammonia concentration or exposure duration). Osmolarity, Cl- and Na+ plasma concentrations were stable in the L and M groups. The increases in Na+, Cl-, K+ and total Ca concentrations observed by the end of the experiment in the H and MH groups suggest that fish failed to adapt. There was all initial rise in plasma cortisol in all ammonia-exposed groups followed by a return to basal level (1.7-4 ngml(-1)) in the L and M groups. In group M H, plasma cortisol peaked at 42 ngml(-1) by day 14, and after a decline at c. 1 month (14 ngml(-1)), it rose again.