The capacity of four different strains of measles virus to adapt to growth in brains of newborn hamsters was studied. The wild Edmonston strain was readily adapted, whereas the further attenuated Schwarz strain (passaged once in VERO cells) was relatively less encephalitogenic in its first brain passage. Among variants of the Edmonston strain that are dependent on the multiplicity of infection, undiluted passage virus caused encephalitis in a high proportion of animals in the first brain passage. A neurotropic isolate of diluted passage virus was obtained only with difficulty. The period of incubation of encephalitis caused by neurotropic undiluted passage virus was significantly longer than those of infections caused by other strains; six to eight days compared with three to six days. Differences were also found with regard to histopathologic changes in animals infected with neurotropic isolates of the different strains. The degree of autointerference displayed by different virus strains in vitro may be one factor, among several others, that determines their neurovirulence.