Relative ventricular mass, percent compact myocardium, total protein, DNA content, and myocyte size were determined for rainbow trout, Salmo gairdneri, ranging in size from 10 to 2000 g. Ventricular mass, ventricular total protein, and DNA content increased linearly with body size. The DNA to protein ratio was reduced slightly over a 100-fold range of body size. Myocyte size increased with heart size. However, the estimated 1.7-fold increase in myocyte volume for a 10-fold increase in heart weight was incompatible with a corresponding 10-fold increase in total protein. Since DNA content increased 10-fold it is suggested that long-term cardiac growth in rainbow trout involves both hyperplasia and hypertrophy.