Studies in the laboratory with salmonids and now in the field with wild salmon clearly show that critical swimming performance has an optimum temperature. This temperature optimum is coincident with maximum aerobic scope and maximum cardiac scope. At a temperature that is higher than this optimum, however, whole animal performance declines abruptly. Evidence is presented here to suggest that this is directly associated with a decline in cardiac scope which limits oxygen supply to tissues. It is further suggested that the decline in maximum cardiac performance could reflect problems with the heart's own oxygen supply. The reasoning behind this suggestion is that, at temperatures at or below the optimum and probably because of a limitation on oxygen diffusion in skeletal muscle during exercise, venous oxygen does not fall below a threshold level during exercise, and so the heart receives just enough oxygen for its own muscular activity via the cardiac circulation (i.e. the venous return to the heart). However, because high temperature favours increased oxygen extraction by skeletal muscle, which consequently lowers venous oxygen, cardiac oxygen supply may become insufficient to meet cardiac oxygen demand. The hypoxic myocardium then cannot maintain cardiac scope and internal oxygen delivery to tissue declines.