Samples from cardiac and skeletal muscle were obtained immediately after death in a case of fatal digitalis intoxication. Intra- and extracellular electrolytes were determined by atomic absorption spectrophotometry. No changes were found in the extracellular electrolytes, and only modest changes were noted in skeletal muscle. In cardiac muscle, however, profound electrolyte disturbances were recorded with an inward shift of sodium and an outward shift fo potassium. This signifies an extensive and early inhibition of the membrane ATPase in cardiac muscle and stresses the importance of immediate and vigorous treatment of the intracellular electrolyte disturbances with a view to suppressing serious and potentially lethal arrhythmias.