The cells of the kidney proximal segment of the migrating arctic lamprey, Lampetra japonica, contain particles of the same size, electron-density and intracellular location as particles identified by others as very low-density lipoproteins (VLDL) in the liver and intestine of teleost fishes and lampreys. These particles are synthesized within the cisternae of the smooth endoplasmic reticulum and elements of the Golgi complex. They are transferred to the lateral intercellular space and lamina propria by way of the Golgi vesicles and an intracellular channel system. Some particles are discharged into the lumina of the sinusoidal capillaries of the lamina propria. Although the physiological role of lipoprotein secretion in the renal proximal segment cells is unknown, the present observations provide morphological evidence that the kidney of the arctic lampreys synthesizes lipoproteins and releases them into the circulation at the time when they are undertaking their anadromous migration.