We have evaluated a congenic strain of mice with congenital polycystic kidney disease in which the disease process appears to closely resemble human infantile polycystic kidney disease. Cysts formed first in the proximal tubules of the nephron and appeared, by light microscopy, to be preceded by vacuolization of the cells. These spaces, as seen by electron microscopy, occurred between adjacent cells. The pancreas was severely involved with reduction of both exocrine and endocrine elements. Cyst formation in the liver was minimal. Serum samples evaluated for urea nitrogen and creatinine were significantly elevated in affected mice. Serum glucose was within normal limits.