Reproductive efficiency is the most important economic factor in livestock production. Thus, the hypothalamo-pituitary-gonadal regulatory axis, accessory sexual organ functionality, and the complex events involved in fertilization, implantation, and embryonic and fetal development may be sensitive to therapeutic agents, environmental pollutants, and natural toxicants. There are many factors that adversely affect reproduction, one of which is toxic substances in the diets of animals. Toxic materials can affect reproductive success by causing abortions, interfering with libido, estrus, oogenesis, or spermatogenesis, causing emaciation and subsequent abnormal mating behavior, birth defects, and increasing the time between parturition and rebreeding. Examples of natural toxicants in poisonous plants interfering with reproduction are numerous. Abortion in livestock from locoweeds, ponderosa pine needles, broom snakeweeds, fescue, and others are reported in studies. Selenium and seleniferous forage inhibit estrus in cattle and swine. Emaciation and temporary illness from sneezeweeds, bitterweed, locoweed, larkspur, lupines, and others may interfere with mating. Embryonic loss and birth defects from Veratrum, lupines, locoweeds, poison hemlock, and so on, may occur. As suggested, toxins have many diverse and economically adverse effects on reproductive performance in livestock.