Exposure to lead causes decreased fertility in women. In the present study, we examined the in vitro effects of lead on cytochrome P450 aromatase (P450 arom) and on estrogen receptor β (ERβ), two key proteins for the human ovary. Aromatase is required for the bioconversion of androgen to estradiol; ERβ mediates estrogen effects in granulosa cells. Granulosa cells were collected from women undergoing in vitro fertilization and then cultured with 10 µM lead acetate. Using atomic absorption spectrometry, we showed that lead accumulated in cells. Aromatase activity as measured by a tritiated water production assay was significantly reduced. Using semiquantitative reverse transcription-polymerase chain reaction and Western blotting procedures, we showed that P450 arom and ERβ mRNA and protein content were both significantly reduced. Adding 10 µg/ml of cycloheximide, a protein inhibitor, did not eliminate the effects of lead. The present results support the hypothesis that the action of lead on fertility in women may result, in part, from the down-regulation of P450 arom and ERβ gene transcription in ovarian granulosa.