The development of hydrocephalus has been investigated in a strain of mice with an autosomal recessive gene for hydrocephalus. The hydrocephalus is apparent in histological sections by 1 day after birth as a dilatation of the lateral ventricles and cavitation of the surrounding brain. Infusion experiments show that the flow pathway out of the ventricles is obstructed in 4 day hydrocephalic animals or older but not in newborn animals. The resistance to absorption of the cerebrospinal fluid from the lateral ventricles is high in normal animals at 1-2 days after birth and it falls to adult values at 14 days. Resistance from the cisterna magna is lower than from the lateral ventricles in animals less than 14 days suggesting that, in the first week or two after birth, there is a resistance in the flow pathway through the ventricular system. In hydrocephalic animals the resistance of the flow pathway was higher than normal. There was no defect in the capacity for fluid absorption from the subarachnoid space in hydrocephalic animals.