1. Chronic administration of frusemide in large doses of 4 mg day−1 kg−1 for 3 weeks caused a significant reduction of cell water in rabbit cardiac and skeletal muscle. Intracellular Na+ concentration, intracellular pH and extracellular space were unchanged in both tissues. Intracellular K+ concentration increased slightly in both cardiac and skeletal muscle.
2. It is concluded that frusemide does not reduce intracellular K+ concentration in cardiac or skeletal muscle of normal animals receiving a normal oral potassium intake.