Treatment of eggs with the steroid mestranol (3-Methoxy-17a-ethynylestra-1,3,5(10)-trien17pi-ol) as a spray on eggs is a potential avian chemosterilization procedure. Eggs of Japanese quail (Coturnix coturnix) were treated topically at 0, 6, or 12 days of incubation with a 0.005-ml drop of fuel oil containing 2.5, 5, or 10 ,ug of mestranol, or before incubation with a 0.005-ml fuel oil spray containing about 5 ,ug of mestranol. Controls for each treatment were treated with fuel oil only. Quail hatched from mestranol-treated eggs were irreversibly sterile and sometimes anatomically intersexual. Birds from treated eggs sometimes had significantly (P < 0.005) heavier ovaries and frequently had smaller oviducts and cloacal glands than the controls. When high levels of mestranol were given early in incubation, there was also higher embryo and chick mortality. Females had higher mortality and lower fecundity than males. Fecundity data, in conjunction with weight data and the histology of secretory tissue, suggest permanent changes in the hypothalmic centers of treated birds. Wild birds, like insects and mammals, may conflict with man's economy and welfare. There are several bird populations that are classified as being a nuisance. Control of some nuisance insect populations is more effective when a sterilization program, rather than induced mortality, is initiated and this method of control might be applicable for nuisance avian populations (Knipling 1964, 1968:7-40). The objective of this research was to test the steroid mestranol (3-Methoxy-17a-ethynylestra-1,3, 5(10)-trien-173-ol), a potential avian chemosterilant, to determine its effect on avian fecundity in controlled experiments using a very prolific pilot species, the Japanese quail. In our experimentation, application of mestranol to eggs in drop form or as a spray is a modification of dipping and injecting procedures used by other workers in studies of sex reversal, bursectomy, and 1 This is a contribution of the Massachusetts Cooperative Wildlife Research Unit (supported by the U. S. Bureau of Sport Fisheries and Wildlife, the Massachusetts Division of Fisheries and Game, the University of Massachusetts, and the Wildlife Management Institute) and the Massachusetts Agricultural Experiment Station. treatment of eggs with antibiotics. Snedecor (1949) presented original results and reviewed other work on the development of chick embryos injected with sex hormones. He reported varying degrees of sex reversal following the use of several estrogenic steroids injected at different dose levels into eggs. Following the claims presented in a patent by Seltzer (1956) involving the dipping of eggs into estrogenic solutions, additional experiments with chick sex reversal were conducted (Van Tienhoven 1957, Mellen 1957, Pincus and Hopkins 1958, Kondo 1959, and Pincus and Erickson 1962). These investigators found changes in both the primary and secondary sex tissue of birds hatched from treated eggs with some birds showing an alteration in reproductive performance. We are grateful to Drs. D. E. Davis, Department of Zoology, North Carolina State University, Raleigh; R. I. Dorfman, Syntex Research, Palo Alto, California; E. F. Knipling, U. S. Department of Agriculture, Beltsville, Maryland; and J. G. Snedecor, Department of Zoology, University of Massachusetts, Amherst; for their advice and encouragement during this