Hydrocephalic mutant mice and matched siblings at different ages were used to measure the pressure and the resistance to drainage of the cerebrospinal fluid (CSF) from the lateral ventricles and from the cisterna magna with glass micropipets. The resting CSF pressure in normal mice increased between 1 to 2 and 4 to 8 days after birth and subsequently decreased between 4 to 8 and 14 days after birth. In hydrocephalic mice the resting pressure was not significantly different from normal in the 1st week after birth, but by 14 days the pressure was significantly higher in hydrocephalic mice. For normal mice, the resistance from the lateral ventricles at 1 to 2 days after birth was 143.9 mm H2O min microliter-1 and it decreased rapidly to 62.0 at 4 to 8 days, and to 21.2 mm H2O min microliter-1 at 14 days. The resistance to absorption from the cisterna magna in normal mice declined from 94.9 to 44.4 and to 26.8 mm H2O min microliter-1 at 1 to 2 days, 4 to 8 days, and 14 days after birth, respectively, suggesting that the absorptive capacity of the subarachnoid outflow sites increased during that period. Thus resistance measured from the lateral ventricles was significantly higher than that from the cisterna magna in the 1st week after birth, suggesting that in immature mice there is a resistance to flow of CSF through the ventricular system. In hydrocephalic mice the resistance measured from the lateral ventricles was higher than for normal animals at 181.5, 106.4, and 103.7 mm H2O min microliter-1 for 1 to 2 days, 4 to 8 days, and 14 days, respectively. Resistance from the cisterna magna in hydrocephalic animals was not significantly different from normal at any age. Thus it is concluded that the hydrocephalus is associated with an obstruction to the flow of CSF from the ventricles.