Technical Abstract: Conditioned food aversion is a powerful experimental tool to modify animal diets. We have taken it from an experimental tool to a management tool to prevent livestock from grazing poisonous plants like larkspur, locoweed, and ponderosa pine on western U.S. rangelands. The following principles pertain to creating and maintaining aversions on rangeland:; animals are fed the target plant in a pen, then are dosed with an emetic to induce GI distress; lithium chloride (LiCl) is the most effective emetic and the optimum dose for cattle is 200 mg/kg BW and 150 mg/kg BW for sheep; mature animals retain aversions better than young animals; novelty of the plant is important, although aversions can be created to familiar plants; and averted animals should be grazed separately from non-averted animals to avoid the influence of social facilitation which can rapidly extinguish aversions. Cattle that have been averted to tall larkspur and locoweed have abstained from eating these plants in extensive, ranch-scale demonstrations. Aversive conditioning may provide an effective management tool to prevent animals from eating palatable poisonous plants that cause major economic loss.