The optimal time for treating congenital hydrocephalus is still unknown. The following study tries to answer this question by shunting hydrocephalic rats around the 10th day of life when the brain development corresponds to that of humans at birth. The animals were allocated to 4 groups: normal, hydrocephalic, normal sham-operated and hydrocephalic-shunted rats (groups 1-4). Body weight and neurological development was tested every 2nd-3rd day. X-rays of the skull were performed in groups 2, 3 and 4 at 10 days and in all at 20 days. In groups 2 and 4 0.1 ml lohexolum was instilled into the lateral ventricle at 10 days and in all at 20 days. X-rays were used to calculate the volume of the neurocranium and to identify ventriculomegaly and its grade (1-3). The x-rays and shunting were performed under general anesthesia, the latter using Kausch's technique. At 20 days of age the animals were sacrificed, the homogenized brain tissue was used to measure Protein, DNA, GAD and CNP by photometry, fluorometry and enzymatically. Time-course of neurological development in group 1: The locomotor patterns were attained gradually up to the ability to hold on to a bar. In group 2 there were larger variations and holding on to a bar was possible only in 6/13. In addition, a distinct tremor was observed in 34%. In group 4 holding on to a bar was possible in all, tremor was observed in 20% of examinations and diminishing following surgery. The volume of the neurocranium in groups 2 and 4 was significantly larger than in groups 3 at 10 days; and in group 2 significantly larger than in groups 1, 3 and 4 at 20 days. In group 4 the ventriculomegaly was distinctly reduced in 7 and remained the same in 1. Protein was significantly lower in group 4. DNA was the same in all groups. GAD was the lowest and the CNP significantly low in group 2. The study shows that an effective shunt may reverse the progressive clinical, radiological and neurochemical changes of hydrocephalus, and that surgery must not be performed necessarily earlier (e.g. prenatally in humans).