The opisthonephric kidney of larval anadromous sea lamprey, Petromyzon marinus L., undergoes a programmed regression during metamorphosis. Degeneration is initiated in the anterior end of each kidney and progresses posteriorly until the kidneys are reduced to short, pigmented strands by the end of metamorphosis. The first sign of degeneration in both the epithelium of the renal corpuscles and the tubules is a folding of the basal lamina. Autolysis then occurs throughout the entire epithelium of the nephron with the gradual accumulation of larger and greater numbers of acid phosphatase-containing autophagic vacuoles, cytosomes, and myelin figures. Cytoplasmic debris and electron-dense material accumulates in the tubular lumina and in the urinary space. Although no definitive evidence is provided for the method of removal of the tubular epithelium, macrophages play a large part in the phagocytosis of the components of the renal corpuscle. Mesangial cells appear to engulf debris from the capillaries while a second type of macrophage is involved in the destruction of podocytes and parietal epithelial cells. The method of programmed degeneration of the renal corpuscle closely resembles descriptions of the mammalian renal corpuscle in diseased conditions. The sole surviving element of the degeneration of the entire nephron epithelium is a pleated basal lamina. The regressing larval opisthonephros has potential as an alternative system for studying a normal developmental pattern such as tissue regression.