A single intracisternal injection of 0.4 ml of 1.25 and 2.5 percent gentamicin sulfate with preservative to healthy adult rabbits caused acute respiratory paralysis and severe seizure activity initially and paralysis of the limbs subsequently. In the white matter of the upper cervical spinal cord, multiple, minute, disseminated, spongy lesions were observed. They consisted of lysis of axis cylinders, edematous dilatation of myelin sheaths, and loss of astroglia and interfascicular oligodendroglia. Axonal end-bulbs formed at the periphery of the lesions. Clinically and morphologically, 0.025 and 0.25 percent gentamicin sulfate solution did not produce myelopathy. The spinal lesions were distributed differently from those of other chemical myelopathies in that they developed in the deeper white matter with sparing of marginal myelinated fibers. Circumscribed high concentrations of gentamicin, and vulnerability of myelinated axis cylinders and interfascicular oligondendroglia to gentamicin may be the main factors causing these lesions. When gentamicin sulfate without preservative was injected, neutrophil leukocyte infiltration occurred actively in the spongy lesions. In the cervical spinal ganglia some nerve cells underwent cytoplasmic vacuolation. In control animals a single intracisternal injection of saline or preservative did not result in the production of these lesions.