32 guinea pigs were exposed to 100-138 dB wide band noise for 1 min to 1 hour and sacrificed immediately afterwards or after short survival times. In light and electron microscopic examinations the initial structural changes proved to be: distortion of outer hair cells, buckling of sensory hairs, swelling of the dendrites to the inner hair cells, and increase of density in efferent nerve endings at the outer hair cells, followed by disintegration of hair cells and rupture of dendrites with incipient retrograde nerve degeneration at higher exposures. Thxe appears to be a critical intensity for these initial struciural changes, which only partly can be explained on the basis of a purely mechanical effect. Swelling of dendrites seems to be reversible to a certain extent, whereas the buckling of sensory hairs of the inner hair cells remains even after longer survival times.