Female birds model a type of woven bone prior to egg laying which is known as medullary bone. Medullary bone modeling is estrogen dependent and in the female fowl coincides with a decrease in cancellous bone volume. Medullary bone modeling was induced in male laying-strain fowl by the administration of estrogen and prevented in females by the administration of tamoxifen. In estrogen-treated males, medullary bone modeling was accompanied by cancellous bone loss; cancellous bone volume was significantly lower than in control males (P < 0.001). In females, the prevention of medullary bone modeling by tamoxifen treatment resulted in significantly higher cancellous bone volumes than in control females (P < 0.001). Estrogen therefore appears to play a role in cancellous bone loss in the fowl.