Polioencephalomalacia developed in 27 of 225 cattle grazing on 486 hectares of dry, short, grama grass pasture. Chemicals in drinking water, toxin from nitrate-utilizing ruminal bacteria, and documented poisonous plants were considered as etiologic agents. Attempts to reproduce the disease by injecting mice and dosing sheep with broth filtrate from nitrate-utilizing ruminal bacteria were not successful. Mushrooms collected from the pasture and fed to a cow did not reproduce the disease.