Hydrocephalus is associated with brain compression and accumulation of neurotransmitter waste products in the brain and cerebrospinal fluid. We postulated that the extracellular compartment is compressed and specifically hypothesized that extracellular fluid tracer movement through brain would differ between control and hydrocephalic rats. Gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA) was injected into the cerebral cortex of 4-week-old rats, 7-11 days after induction of hydrocephalus by kaolin injection into the cisterna magna. The movement of this soluble paramagnetic compound was followed over successive timed intervals from 20 min to 180 min with T1-weighted magnetic resonance imaging. Non-hydrocephalic controls exhibited greater spread of the tracer and greater change in T1-weighted signal intensity in the ipsilateral cortex than hydrocephalic animals. Hydrocephalic animals exhibited preferential accumulation of tracer in edematous white matter. Gd-DTPA penetrated the lateral ventricles within 30 min in both control and hydrocephalic rats. The results suggest that there is a relative impairment of extracellular fluid movement through the cerebral cortex of young hydrocephalic rats.