Extracellular space (ECS), water, and intracellular ion concentration were determined in the normal and hypertrophied rat myocardium. Theoretical measurements of myocardial ECS based on a mathematical model (9.31%) closely approximated the observed results in the normal myocardium (12.8%) and hypertrophied myocardium (11.7%). Observed results confirmed theoretical considerations of no change in ECS with hypertrophy. ECS and water content were higher in the right ventricle than the left ventricle, but intracellular electrolytes remained unchanged. Myocardial hypertrophy did not alter these relationships.