The neuronal basis for neurological deficits in infantile hydrocephalus is poorly understood. Changes in the dendritic architecture of pyramidal cells of the auditory cortex have been measured at 21 days after birth in H-Tx rats. Tissue was prepared by the rapid Golgi method from hydrocephalic and control littermates, together with hydrocephalic rats with ventriculo-subcutaneous shunts placed at 3-4 days or at 10 days after birth. Layer V pyramidal cells were analyzed quantitatively on a light microscope at a magnification of 250 or 400x. When compared to control, the hydrocephalic rats had a 30% reduction in the cortical thickness whereas in the shunt-treated rats it was similar to control. For both the apical and the basal dendrites, the distance extended from the soma was reduced in hydrocephalic rats by 49-57%, and the total length of the dendritic trees was decreased by 61 and 77%, respectively. Rats shunt-treated at 3-4 days had small dendrite changes which, in most cases, were not significantly different from control. Rats shunt-treated at 10 days had dendrites which were indistinguishable from untreated hydrocephalic rats. Dendritic branch patterns were also affected; the number and mean length of branch segments were reduced in both the hydrocephalic and the 10-day shunt group, with only small changes in the earlier group. Overall, the basal dendrites were more severely affected than the apical dendrites. It is concluded that infantile hydrocephalus results in severe neuronal abnormalities which can largely, but not completely, be prevented by shunt treatment performed in the early stages.