Atlantic salmon postsmolts (0.6-0.7 kg) were exposed to 4 different concentration intervals of un-ionized ammonia nitrogen (NH₃-N) and total ammonia nitrogen (TAN=NH₄⁺-N+NH₃-N) in an open flow system: 0.2-3 µg/litre NH₃-N and 0.02-0.25 mg/litre TAN (control), 9-17 µg/litre NH₃-N and 1.12-1.41 mg/litre TAN (low), 14-32 µg/litre NH₃-N and 1.80-2.70 mg/litre NH₃-N (medium), and finally 43-80 µg/litre NH₃-N and 5.50-6.70 mg/litre TAN (high). The water temperature was 8-9°C and salinity was 34-35%. After 1 month the mean plasma glucose levels were significantly increased both for the medium and high ammonia group when compared to control. However, the increase was small and the mean plasma glucose values were considered to be in the normal range for Atlantic salmon postsmolts for all groups. No significant differences were found in hematocrit, in plasma chloride, in the growth parameters (weight, length and condition factor) and no differences were observed in the gill epithelium of the groups. The exposure period was only 1 month and further research is needed to study safe levels of ammonia for Atlantic salmon postsmolts. In such studies plasma glucose seems to be an important parameter to measure. However, under less controllable conditions other factors than ammonia may increase the plasma glucose level.