Aim To develop an aromatase inhibitor screening a ssa y in vitro. Methods A method termed “tritiated water”assa y was developed to measure aromat aseactivity by quantifying the 3H 2O from the aromatization reaction, us ing placental microsomes as enzyme source and [1?- 3H]androstenedione as a substrate. Result “Tritiated water” assay was establishe d and it was found that androstrenedione had a K m value of 113 nmol?L -1 and a V max value of 33.3 pmol?(min?mg protein) -1 in the aromatiz ation reaction catalyzed by placental microsomes as determined by this assay. The reliability of t his assay was confirmed by application of specific aromatase inhibitor exemestan e. Exemestane was shown to be an irreversible aromatase inhibitor with the IC 50 of 70 nmol?L -1, which was comparable to findings by other laboratories.Conclusion These data demonstrate that “tritiated water” assay is a useful model to screen aromatase inhibitors.