Infant and weanling mice infected experimentally with non-neuroadapted Western equine encephalitis virus exhibited a differing spectrum of target-organ pathology as well as a different clinical response. The infant mice sickened and died within 48 hr. Pathological changes in this group were largely limited to such mesodermally derived tissues as muscle, cartilage, and bone marrow elements. The central nervous system was uninvolved, as were heart, lung, liver, and other body organs. The young adult mice, on the other hand, showed no clinical evidence of disease throughout the 376-hr postinfection period. Nevertheless, after 216 hr they developed acute diffuse meningoenceophalitis of the viral type, with concomitant changes in heart, lung, liver, kidney, and brown fat. The most striking and consistent finding in the infant group was acute necrotizing degeneration of the bone marrow. The most dramatic change in the young adult group was acute necrotizing meningoencephalitis.