The male offspring of Long-Evans rats treated with the aromatization inhibitor ATD (1,4,6-androstatriene-3,17-dione) during pregnancy show high levels of lordotic behavior when treated with estrogen and progesterone in adulthood. The male offspring of Sprague-Dawley dams treated in the same way show only a slight facilitation of lordotic potential. These strain differences could reflect strain differences in gestation length and therefore differences in the timing of the sensitive period of sexual differentiation; they could reflect differences in the sensitivity to the defeminizing actions of gonadal hormones; or they could reflect differences in the sensitivity to ATD treatment. We therefore directly compared the effects of prenatal and early postnatal treatment with ATD on the potential of male Long-Evans and Sprague-Dawley rats to show lordosis when given estrogen and progesterone in adulthood. In both strains ATD treatment facilitated adult lordotic behavior. Treatment appeared to have a greater effect in the Long-Evans strain. However, control Long-Evans males were substantially more responsive to hormone treatment in adulthood than were Sprague-Dawley males. In the Long-Evans strain short-term ATD treatment (Days 20-23 of pregnancy) was as effective as long-term treatment (Days 10-23). In the Sprague-Dawley strain, ATD treatment was most effective when given prenatally and postnatally. Strain differences in hormonal sensitivity best account for the present findings.