Trout that had been held in freshwater or in sea water were stored at 0, 5, or 10 °C, and in the case of sea-water-held trout, also at 15 °C. Samples were taken during storage for analysis of ATP-derived metabolites. The kinetics of degradation of ATP were investigated using two mathematical models, one depending on only endogenous enzymes acting in a sequence of consecutive first order reactions, and one assuming inosine was additionally converted to hypoxanthine by bacterial action. The former model adequately fitted the data from trout held in sea water, but the latter model was a better fit to data from the trout held in freshwater. The activation energy of loss of inosine monophosphate was estimated to be 17.4 kcal mol−1.