Chronic effects of ammonia were studied in juvenile seabass, Dicentrarchus labrax (mean weight= 11 g), exposed for 63 days to eight stable ammonia concentrations, ranging from 0.24 to 0.90 mg l(-1) unionised ammonia nitrogen (UIA-N), respectively, from 6.1 to 22.3 mg l(-1) total ammonia nitrogen (TA-N). Temperature (21.8 degreesC), pH (8.0), salinity (37.0 ppt), and oxygen concentration (over 80% saturation at the outlet) were maintained constant. Fish were fed using a self-feeder device, and they were starved during the last 8 days. Mortality of 28.9 and 42.6% occur-red within the first 8 days at the two highest UIA-N concentrations, respectively, 0.90 and 0.88 mg l(-1). From days 0 to 55, a 1.8-fold increase in weight gain was observed under the 0.90-mg l(-1) UIA-N condition, compared to a 3.4-fold increase in the control. Weight gains were negatively correlated to ambient ammonia concentrations. Weight loss, or a transient period of growth stagnation, was observed from the onset of ammonia exposure to day 13 in seabass exposed to concentrations above 0.43 mg l(-1) UIA-N. After day 13, weight gains were observed in all groups, indicating that the fish were able to adapt to increased ambient ammonia concentrations over time. By the end of the experiment, plasma ammonia levels were positively related to ambient ammonia concentrations, and oxygen consumption recorded in fasting fish was significantly dependent on ammonia concentrations. In seabass juveniles, the 0.26-mg l(-1) UIA-N concentration, under an average pH of 8.0, can be considered as a safe long-term limit conditions in seawater.