Aim: To assess whether exogenous estradiol has any effect on migration of primordial germ cells (PGCs) in the chick. Methods: Fertilized eggs were treated with 17beta-estradiol (Ez) (80 lag/egg) at stage X (day 0 of incubation),stages 8-10 (incubation 30 h) and 13-15 (incubation 55 h). Controls received vehicle (emulsion) only. Changes in PGC number were measured on different days according to developmental stages. Results: In male right gonads,but not in female left gonads, at stages 28-30 (incubation 132 h) significant decreases in the mean number of PGCs aggregating were observed compared with the controls (P 0.05). Conclusion: The present study provides evidence that E2 has significant effects on the localization of PGCs in male fight, but not female left, gonads of chicken embryos at stages28-30, compared with controls.