The breakdown products of adenosine triphosphate (ATP) and the K-value were determined as a means of evaluating the quality of some heated fishery products. During hot smoking, the K-value usually increased by 6 to 30%. Duringsous vide-treatment of whitefish, the K-value increased from 4 to 48%. Nucleotide breakdown did not occur to a marked extent in hot smoked orsous vide-treated products during subsequent storage. The denaturation of water-soluble proteins was monitored by electrophoretic methods. The enzymes that degrade nucleotides as well as soluble proteins of rainbow trout were denatured at 65 to 70 °C. The method could be tentatively used for the evaluation of what the level of freshness of the rainbow trout was before hot smoking. The K-value of rainbow trout did not increase during cold smoking but did increase afterwards during subsequent cold storage, in a fashion similar to that occurring with raw fish. Differences in the contents of nucleotide breakdown products between individual fish samples decrease the suitability of this method.