The toxicity of ammonia to Eriocheir sinensis juveniles was determined. The 24 h-, 48 h-, 72 h-, 96 h-LC(50) values of total ammonia (TAN) were 251.68, 217.61, 156.05, and 119.67 mg L(-1), respectively. Following these results, crabs were then exposed for a 2-day period to 20, 40, 60 and 80 mg L(-1) TAN and sampled at 3, 6, 24 and 48 h for changes in metabolic parameters (including haemolymph ammonia concentration, glucose, lactate, urea, triacylglycerol, glutamine, and glutamate levels) and immunity indicators (the total of haemocyte count and superoxide dismutase activity). Results showed a distinct linear relationship between ambient ammonia and haemolymph ammonia and a notable increase in haemolymph ammonia content after ammonia exposure. Compared with the control group, lower concentration of triglycerides and significantly higher glucose, urea, and lactate level in haemolymph were observed when ambient ammonia increased. This suggested a reduced use of carbohydrates through anaerobic metabolism and an increase in the use of lipids to satisfy the metabolic demand. A significant surge of the ammonia metabolic product, glutamate, was observed after 3 h ammonia exposure, and the compensatory response to reduced glutamate was manifested by increased glutamine synthesis. During the same period, total haemocyte count decreased while ambient ammonia increased. Superoxide dismutase (SOD) activity in haemolymph was stimulated by lower ambient ammonia concentration after short time exposure and depressed by higher ammonia concentration. Therefore, haemolymph ammonia accumulation resulted in an increase in energy demand and a depression in immune capacity. The mechanism to detoxification of ammonia may be to transform ammonia to urea and glutamine.