The basilar artery is one of the three major sources of blood supply to the circle of Willis. To investigate the effects of basilar artery occlusion, we surgically exposed and coagulated the basilar artery in 25 rats. Basilar artery occlusion at any single point between the foramen magnum and the circle of Willis in 11 rats did not produce histologically detectable infarcts in the brain at 12-24 hours. Two-point occlusions of the basilar artery in 12 rats produced variable infarcts between the occlusion sites but no ischemic lesions elsewhere. After either single- or double-point occlusions, the proximal basilar artery refilled within 2-3 minutes. When the basilar artery was occluded above and below the origins of the anterior inferior cerebellar arteries, the artery segments between the occlusion points initially collapsed but refilled within 2-3 minutes in two rats. Basilar artery occlusions invariably suppressed cortical somatosensory evoked potentials by greater than 50%. Regardless of whether a brain stem infarct developed, somatosensory evoked potential amplitudes recovered to greater than baseline levels by 4 hours in seven of 17 rats and returned to baseline levels by 24 hours in every rat tested. We conclude that the occluded basilar artery receives extensive retrograde collateral blood flow and that somatosensory evoked potentials are exquisitely sensitive to basilar artery occlusion but are insensitive to whether brain stem infarcts develop.