Forage plant estrogens occur at high levels in certain forage species. Such levels are responsible for reproductive and other effects in animals. Current progress regarding the chemical, metabolic, and biological effects, as well as occurrence of the several naturally occurring forage estrogens, is described. Forage estrogens occur as natural plant responses to foliar and other plant diseases. Their potency in animals is approximately 1/1000 to 1/10,000 that of the principal animal hormones such as estradiol. Coumestans and isoflavones comprise the two principal classes of forage estrogens, with coumestrol and genistein being the most potent and predominant of each class. Only very low levels of the coumestans occurred in alfalfa leaf protein concentrates prepared at alkaline pH.