Nine adult beagle dogs underwent magnetic resonance imaging in a 2-Tesla small-bore unit. Six surviving dogs were followed up serially with magnetic resonance imaging after induction of hydrocephalus by injection of Silastic into the prepontine cistern or fourth ventricle. Ventricular size (Y) measured as percentage cross-sectional area of an anterior frontal slice was related to postoperative day (X) as Y = 1.54 + 4.21 x ln(X), r = 0.9596. Periventricular edema appeared initially in the superlateral angles of the frontal horns in an area that corresponded histologically to the subcallosal fasciculus. T1 relaxation time of normal white matter of 979.32 msec increased to 1813.90 msec in the area of the edema (p less than 0.0001). The T2 relaxation time of normal white matter of 83.39 msec increased to 238.26 msec in the area of the edema (p less than 0.0001). Histological changes included expansion of the extracellular space in an area comparable to the region of increased signal intensity on T2-weighted images, as well as diffuse astrocytosis in the chronically hydrocephalic dogs.