The increase in volatile nitrogenous bases in codfish muscle between the pre-rigor period and the first appearance of odour is approximately 6 mg. per 100 grams of tissue, and is due almost entirely to the action of bacteria. It can be used to follow the course of spoilage only if the original value of the fish in question is known, as the range in variation of the original values is as great as the increase to the appearance of odour. A method for the rapid determination of "trimethylamine" in cod muscle has been devised. Its increase parallels the increase in bacterial population. Odours always appear at approximately the same level of "trimethylamine". The increase resulting from autolysis is negligible. The increase during the development of spoilage is fifteen to twenty times the original value. Spoilage can be followed in fish preserved with borates as well as in untreated fish.