Kinetics and distribution of K were studied in myocardiuum of chronic Mg-deficient rats. The experimental design was as follows. Rats were fed either Mg-deficient or control diet for 30 days. They were then injected with 0.2 mCi of K-43. One half hour after injection they were killed and a segment of myocardium was washed with nonradioactive Krebs solution in a special chamber. Radioactivity remaining in the tissue was recorded continuously for 1 hour. A three-compartment model (extracellular, main intracellular, and subcellular) was used; transport rate constants (kij) and relative compartment sizes (qj) were determined. In myocardium of Mg-deficient rats the kij's and q's indicated greatly reduced intracellular accumulation of K but a large accumulation of K within the subcellular compartment probably mitochondrial. There was also some enlargement of the extracellular compartment. The extracellular to overall intracellular ratio for K increased 46% in Mg-deficient myocardium. The hypothesis that dietary Mg deficiency affects Na-K pump function is discussed.