Renal function studies were done in five children with infantile polycystic disease (IPCD)of kidneys and liver and in four with congenital hepatic fibrosis (CHF). Glomerular filtration rate was reduced in all IPCD patients and in two of four CHF patients. Urinary concentrating ability following water deprivation and vasopressin administration was impaired in all IPCD patients and in three of four CHF patients. During control period, all patients had asymptomatic metabolic acidosis with total carbon dioxide content less than or equal to 20.5 millimols/liter, and net acid excretion (NAE) was reduced in all but one. Ammonium chloride was administered to seven patients; NAE increased in all, but the increments were subnormal in four. The inability to excrete maximally concentrated urine and an adequate amount of net acid may best be explained by abnormal tubular structure or alterations in medullary architecture secondary to progressive scarring, or both.