The brain of 17 asymptomatic normal adult beagles was evaluated with magnetic resonance imaging. Lateral ventricular size was quantified using computer algorithms. In beagles with large ventricles, contrast medium was introduced into the cerebellomedullary cistern to determine whether obstructive hydrocephalus was present. Total lateral ventricular volume ranged from 77 mm3 to 11,726 mm3. Based on Gd-DTPA diffusion, the CSF flow pattern was considered normal and normal neural morphology was seen using electron microscopy. Results suggest a high incidence of clinically asymptomatic ventriculomegaly in normal adult beagles.