In field observations of young cattle grazing on certain high-altitude ranges infested with the locoweed Oxytropis sericea, a high frequency of clinical congestive right ventricular failure was identified. At necropsy of affected calves, there were right ventricular hypertrophy and dilation, subcutaneous edema, hydrothorax, diarrhea, and chronic passive congestion of liver. Microscopically, there was neurovisceral foamy cytoplasmic vacuolation, characteristic of locoism. An experiment was set up to test the relationship of O sericea to congestive right-ventricular failure. Preliminary investigation supports the field observation.