The effect of temperature and body size on oxygen uptake of European eels,Anguilla anguilla, was studied. The mass specific oxygen uptake of large eels was lower at all temperatures than that of small eels. The effect of temperature on metabolism was greater on small eels than on large eels. The relationship between oxygen consumption and body size is described by the equationM = a Wb, whereMis metabolism (O2h‐1), W is body weight (g),bis the slope of the function which describes the relationship between body weight change and metabolism, andais the temperature‐dependent constant of the equation. In this study it was found that increased temperature caused an increase inaand a decrease