During development, the genotype of the zygote determines the nature of the gonad, which then determines the male or female phenotype. The molecular events underlying this process are just beginning to be defined. A single treatment of chicken embryos with an aromatase inhibitor (which blocks the synthesis of estrogen from testosterone) at a stage when their gonads were bipotential caused genetic females to develop a permanent male phenotype. These sex-reversed females developed bilateral testes that were capable of complete spermatogenesis and had the physical appearance and behavior of normal males. This result identifies aromatase as a key developmental switch in the sex determination of chickens.