Six measles viruses of different origin and history of passage were examined for their neuropathic properties in suckling hamsters and mice. The Edmonston strain, adapted to tissue cultures by numerous passages in human renal and amnion cells, killed 39% of hamsters on primary inoculation. Two attenuated vaccine strains, Edmonston B and Schwarz, failed to produce symptoms of disease in both animal species. Passage of the Schwarz strain in VERO cells increased its titer and was associated with a marked neurotropism for suckling hamsters. Two strains of measles virus, isolated from the brains of patients with subacute sclerosing panencephalitis, were either apathogenic, or their neurotropism was considerably lower than that of the reference Edmonston strain. A measles virus adapted in hamster brain was included in the study for comparison. Its initially high virulence decreased progressively on passage in VERO cells. Several factors may determine neurovirulence of measles virus for different hosts.