The term _loco_ is of Spanish origin and has come to us through the Spanish speaking residents on the cattle raising plains and the Pacific Coast. The word is defined to mean _mad_, _crazy_, _foolish_. It has been applied indiscriminately to a disease in stock manifested by these symptoms, and to a variety of leguminous plants, found growing on the western lands and supposed to cause the disease in question. The plants complained of are Astragalus Hornii, and A. Lentiginosus (Griesbach) in California, A. Mollissimus (Torrey) and Oxytropis Lamberti (Purshiana) in Colorado and New Mexico. Other allied species, and like these found also in the other Rocky Mountain States, Sophora Serecia, Oxytropis Multifloris, O. Deflexa, Malvastrum Coccinium, and Corydalis Aurea var. Occidentalis have been less confidently charged with producing the disease. These plants grow on poor, dry, sandy or gravelly soils, and having great power of resisting drought, are often in fair growth, and present an abundant mass of leaves when surrounding vegetation is withered up. Hence, it is alleged, the animals are driven to use it when nothing else is obtainable and once accustomed to it, the desire for more becomes a veritable craze or neurosis, and the victim searches for it and devours it to the exclusion of other food. The following quotations may serve to illustrate the effects alleged: Among the symptoms first noticed are loss of flesh, general lassitude and impaired vision; later the animal’s brain seems to be affected; it becomes vicious and unmanageable and rapidly loses both flesh and strength. Frequently when approaching some small object it will leap into the air as if to clear a fence. The patient also totters on its limbs and appears as if crazy. After becoming affected it may linger many months, or a year, but usually dies at last from the effects of the complaint. (Dr. Vasey. Report of Dept. of Agriculture, 1884).