In this monograph are reported the author's experiments on the effects of long continued administration of natural and synthetic estrogens to guinea pigs and other rodents. His experiences confirm the work of others, particularly that of Lipschutz of Chile, in that he was able to produce extragenital fibromas in the domestic guinea pig by the implantation of pellets of natural estrogens as well as diethylstilbestrol. He observed a transient loss of hair in some of his animals. Others failed to gain weight at the same rate as the controls. Noteworthy local and systemic effects of long continued estrogenic stimulation were not obvious. The experiments were not numerous but they were carefully planned and executed. There is a complete bibliography.