The brown discolouration which occurs on heating (120 °C., 1 hr.) white flesh of fish was found to be due largely to reactions of the Maillard type. Browning did not occur in leached flesh unless compounds containing a free aldehyde group, or a potential free aldehyde group, were added. Addition of amino acids and substances which did not possess a free aldehyde group did not occasion browning in leached flesh. Discolouration was inhibited by bisulphite and hydroxylamine and slightly by acids, but it was not affected by heating in vacuo, by storing whole fish in ice prior to heating or by addition of metal ions.