Technical Abstract: In the past, livestock poisoning occurred principally on over grazed ranges. L.A. Stoddart stated "livestock poisoning is natures' sign of a sick range." Retrogression following misuse was the greatest single factor contributing to livestock poisoning. Poisonous plants increased with declining range condition and livestock were forced to eat these poisonous species because of a shortage of other forage. The level of management on most western rangelands has improved, resulting in marked improvement in range condition; yet losses to poisonous plants still occur. Some poisonous species are major components of the pristine, pre-European plant communities tall larkspur, Veratrum, water hemlock, bracken fern, chokecherry, ponderosa pine, and various oak species. Although populations of many poisonous seral increaser species have declined with better management, they are still components of plant communities and fluctuate with changing precipitation patterns locoweed, lupine, death camas, snakeweed, threadleaf groundsel, low larkspur, timber milkvetch, redstem peavine, western bitterweed, orange sneezeweed twin leaf senna, and white snakeroot. Many of the alien invader species are poisonous halogeton, St. Johnswort, poison hemlock, tansy ragwort, hounds tongue, leafy spurge, yellow star thistle and other knapweeds. Poisoning occurs when livestock consume these plants because they are either relatively more palatable than the associated forage, or from management mistakes of running short of desirable forage.