Using 0.45 mol H22O (so-called "heavy" water, deuterium oxide) per 100 M total water, and calculating exchange in the fish body by change in wt. of fish and water, a relatively rapid exchange with the ambient water was found. With small goldfish equilibrium was reached in a few hrs.; with increase in size, the rate of exchange decreased. The exchange was slower with dead fish but still marked. H bound in the carbohydrate of the fish participated little, if at all, in the exchange.