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Sex determination in egg-laying amniotes may be fundamentally different from that of placental mammals. The mammalian ovary differentiates normally in the absence of estrogen, whereas estrogen seems to be crucial for proper ovarian development in birds, reptiles, and lower vertebrates. Estrogens are produced normally by the biosynthetic conversion of androgens by the enzyme aromatase (CYP19), which is the sole mediator of this reaction. Aromatase inhibitors are capable of reversing females to males in turtles and chickens; therefore, a role for aromatase as the female sex determinant has been postulated for species in which sex determination is temperature-dependent. The entire aromatase coding sequence (1,509 base pairs) from adult terrapin ovaries was cloned, and Northern analysis indicates a single transcript (2.4 kb) for adult ovaries, whereas male and female brains express a 2.4-kb as well as a 9.6-kb transcript. Using a sensitive (attomole sensitivity) competitive RT-PCR technique, aromatase transcript abundance was quantified during embryonic development for embryos treated with and without estrogen. Aromatase is transcribed, well before the temperature-sensitive, (stage 12), at both male and female temperatures in the brain. There is a switch to lower aromatase transcript abundance in the female brain concurrent with an exponential rise of aromatase transcript in the putative ovary. Transcripts remain below the detection limits in the putative testes but exhibit female levels of aromatase transcript when treated with estrogen. Aromatase mRNA levels are generally reduced in the brain by estradiol application. On the basis of these findings, we have postulated a model based on the competition between 5 alpha-reductase and P450 aromatase for androgen substrate in both the brain and the undifferentiated gonad to explain the TSD phenomenon in reptiles.