Cows and ewes fed estrogenic forage may suffer impaired ovarian function, often accompanied by reduced conception rates and increased embryonic loss. Males are relatively unaffected, but the mammary glands in females and castrate males may undergo hypertrophy of the duct epithelium, accompanied by secretion of clear or milky fluid. In cows, clinical signs resemble those associated with cystic ovaries. The infertility is temporary, normally resolving within 1 mo after removal from the estrogenic feed. However, ewes exposed to estrogen for prolonged periods may suffer a second form of infertility that is permanent, caused by developmental actions of estrogen during adult life. The cervix becomes defeminized and loses its ability to store spermatozoa, so conception rates are reduced, although ovarian function remains normal. Importantly, both temporary and permanent infertility in ewes often occur without observable signs and can be detected only by measurement of phytoestrogens in the diet, or measurement of their effects on the animal. Low background concentrations of dietary phytoestrogens are suggested to play an important role in prevention of disease in humans and laboratory rats, but subclinical effects of phytoestrogens in cattle have not yet been described. Effects of low concentrations of phytoestrogens on reproductive function in ruminants are likely to receive increasing attention.