Hydrocephalus causes damage to periventricular white matter at least in part through chronic ischemia. Magnesium sulfate (MgSO 4 ) has been shown to be protective in various models of neurologic injury. We hypothesized that this agent would ameliorate the effects of experimental childhood-onset hydrocephalus. Hydrocephalus was induced in 3-and 4-wk-old rats by injection of kaolin into the cisterna magna. Tests of cognitive and motor function were performed on a weekly basis. In a blinded and randomized manner, MgSO 4 was administered in two separate experiments (s.c. injection 0.85, 4.1, or 8.2 mM/kg/d), supplemented by osmotic minipump infusion (0.03 mM/d) to prevent low trough levels for 2 wk, beginning 2 wk after induction of hydrocephalus. The brains were then subjected to histopathological and biochemical analyses. With the 4.1 mM/ kg/d dose, serum Mg ϩϩ levels were elevated transiently from 1.3 to~7 mM/L. We observed statistically significant improvement in gait performance and reduced astroglial reaction. There was also a trend to improved memory performance, but no evidence of increased myelin or synaptic protein content. Hydrocephalus is a common neurologic condition characterized by pathologic dilation of the cerebral ventricles. It is usually caused by obstruction of cerebrospinal fluid (CSF) flow. Axon damage in the periventricular white matter is one of the earliest pathologic consequences of ventricular dilation in humans and animals (1). The pathophysiology of hydrocephalus-induced brain damage is multifactorial, with contributions made by gradual physical stretching, compression, ischemia and possible accumulation of metabolic waste products (2). It has been postulated that physical trauma and ischemic injury to axons combine to alter membrane permeability leading to local influx of calcium and activation of calpains, which can cause proteolytic damage to the axonal cytoskeleton (3).Magnesium is a calcium antagonist that blocks NMDA channel receptors in a voltage dependent manner, as well as voltage and receptor-operated calcium channels (4, 5). Magnesium sulfate therapy reduces lipid peroxidation after experimental spinal cord injury (6), long-term hypoxic-ischemic brain damage in 7-d-old rat (7), and the volume of traumatic brain lesions (8). It also facilitates recovery of function following electrolytic cortical injury (9). Magnesium may act as a cerebral vasodilator thereby improving the cerebral circulation (10) and preventing cell damage. Clinically, magnesium sulfate is used for seizure prophylaxis in the management of pregnant women with eclampsia. Some reports suggest that this treatment is associated with reduced incidence of cerebral palsy in low birth weight infants (11).In an experimental model of hydrocephalus induced by injection of kaolin into the cisterna magna of immature rats we have previously demonstrated the presence of abnormal quantities of soluble ionic calcium and activated calpains in the periventricular white matter (3). Diminished blood flow has been demonstrated in ...