Shrimps Palaemonetes varians were submitted to lethal and sublethal concentrations of ammonia in order to measure lethality, ATP and adenylate nucleotide levels, and adenylate energy charge (AEC) index. LC50 was of about 3 mg/liter of ammonia. Adenylate measurements were performed over a period of 14 days and for two different concentrations. A population submitted to 0.5 mg/liter of ammonia exhibited high survival and a marked consumption of ATP, whereas high mortality and disordered ATP metabolism are the characteristics of the population submitted to 3 mg/liter of ammonia. An homeostatic model was applied in order to explain the significance of the AEC signal. Here, AEC is proposed as a measure of the limits of active response of an organism to environmental stress.