Treatment of developing embryos of two diverse species of reptiles with fadrozole (a potent and specific nonsteroidal inhibitor of aromatase activity in mammals) resulted in the induction of male sex determination. In the first experiment, males were produced in an all-female parthenogenic species of lizard (Cnemidophorus uniparens). In the second experiment, male sex determination was induced in a turtle (Trachemys scripta) with temperature- dependent sex determination. The results support the hypothesis that the endogenous production of oestrogen may represent a pivotal step in the sex determination cascade of reptiles. Further, the production of male C uniparens indicates that the genes required for male sexual differentiation have not been lost in this parthenogenic lizard.