The severity and progression of ventricular enlargement, the occurrence of cerebral edema, and the localization of isch-emic metabolic changes were investigated in a rat model of hydrocephalus, using in vivo1H MR spectroscopic imaging (SI) and diffusion weighted MRI (DW MRI). Hydrocephalic rats were studied 1, 2, 4, and 8 weeks after injection of kaolin into the cisterna magna. Parametric images of the apparent diffusion coefficient (ADC) revealed a varying degree of ventricu-lomegaly in all rats, with different time courses of ventricular explansion. Extracellular white matter edema was observed during the early stages of hydrocephalus, most extensively in cases of progressive ventriculomegaly. In gray matter regions, ADC values were not changed, compared with controls. In case of fatal hydrocephalus, high lactate levels were observed throughout the whole brain. In all other rats, at all time points after kaolin injection, lactate was detected only in voxels containing cerebrospinal fluid. This suggests accumulation of lactate in the ventricles, and/or an ongoing peri ventricular production of lactate as a consequence of cerebral ischemia in experimental hydrocephalus.