After 7 or 30 days of exposure to general vibration (frequency: 40 Hz, amplitude: 1 mm) for 3 hours per day, the prosencephalon of guinea pigs exhibited a wide range of morphological changes, including congestion, haemorrhages, the presence of the heterochromic neurons and perivascular spaces as well as diffuse spongiosis. In contrast, after 14 days' exposure to general vibration of the same parameters, diffuse spongiosis of the prosencephalon was found in all the animals under examination. Hypothalamus, cerebral cortex and periventricular gray matter were unaffected by spongiosis in most of the animals. The nerve fibre sheats were unchanged in all cases.