In order to provide an ultrastructural description of kaliopenic nephropathy in rats, potassium deficiency uncomplicated by chloride depletion or extracellular fluid volume expansion was induced by dietary means alone. After 12 days, serum and muscle analyses demonstrated severe potassium depletion, while urine concentrating ability was markedly impaired. Although micropuncture techniques demonstrated that net fluid transport in proximal tubules was normal, the epithelial cells showed the following morphological alterations: electron-lucent vacuoles and lysosome-like dense bodies were numerous, focal cytoplasmic degradation was present, and intercellular spaces were dilated in many proximal tubules. Medullary cell droplets were characterized by fusion, massive enlargement with sequestration of cytoplasmic components such as ribosomes, and occasional extrusion through ruptured plasma membranes – features not previously reporte