Exposure to microgravity causes alterations in postural, locomotor and oculomotor functions. The vestibular abnormalities experienced by astronauts entail immediate reflex motor responses, including postural illusions, sensations of rotation, nystagmus, dizziness and vertigo, as well as space motion sickness. Adaptation to the microgravity environment usually occurs within one week, and a subsequent re-adaptation period of several months is often required upon return to Earth. Some astronauts experience recurrences of dizziness, nausea, and vomiting, as well as marked disturbances in postural equilibrium in the absence of vision during this readaptation period. The mechanisms underlying such adaptation processes remain unclear, although current evidence favors some type of sensory conflict. The purpose of the present study was to explore the structural basis for the reorganization in the central vestibular system that underlies the process of adaptation to altered gravitational environments. Hindbrain tissue was obtained from rats flown on the Neurolab shuttle mission (STS-90) that launched on April 17, 1998. Tissue for the present report was obtained from four adult Fisher 344 rats sacrificed on orbit during flight day 2 (FD2), 24 hr after launch. Equal numbers of vivarium control animals and cage-controls were sacrificed 48 and 96 hr, respectively, after the flight dissections. Following decapitation, each hindbrain was immersion-fixed for 45 min in 4% paraformaldehyde/0.1% glutaraldehyde in 0.1M phosphate buffer pH 7.3, and then transferred to a 4% paraformaldehyde solution in 0.1M phosphate buffer for 18 days at 4 degrees C. After this fixation, the cerebellum was dissected away from the ventral portion of the brainstem by severing the cerebellar peduncles. The entire cerebellum of each rat was cut by Vibratome into 100 micrometers thick sections in the parasagittal plane. These sections were collected serially and processed for electron microscopy by osmication, dehydration in a graded series of methanol solutions, infiltration with resin, and embedment in Epon-Araldite resin between plastic coverslips.