estrogens , whether natural or synthetic , clearly influence reproductive development , senescence , and carcinogenesis . 
 pyrethroid insecticides are now the most widely used agents for indoor pest control , providing potential for human exposure . using the mcf-7 human breast carcinoma cell line 
 , we studied the estrogenic potential of several synthetic pyrethroid compounds in vitro using ps2 mrna levels as the end point . 
 we tested sumithrin , fenvalerate , d - trans allethrin , and permethrin . 
 nanomolar concentrations of either sumithrin or fenvalerate were sufficient to increase ps2 expression slightly above basal levels . at micromolar concentrations , these two pyrethroid compounds induced ps2 expression to levels comparable to those elicited by 10 nm 17ss - estradiol ( fivefold ) . 
 the estrogenic activity of sumithrin was abolished with co - treatment with an antiestrogen ( ici 164,384 ) , whereas estrogenic activity of fenvalerate was not significantly diminished with antiestrogen co - treatment . 
 in addition , both sumithrin and fenvalerate were able to induce cell proliferation of mcf-7 cells in a dose - response fashion . 
 neither permethrin nor d - trans allethrin affected ps2 expression . 
 permethrin had a noticeable effect on cell proliferation at 100 microm , whereas d - trans allethrin slightly induced mcf-7 cell proliferation at 10 microm , but was toxic at higher concentrations . 
 overall , our studies imply that each pyrethroid compound is unique in its ability to influence several cellular pathways . 
 these findings suggest that pyrethroids should be considered to be hormone disruptors , and their potential to affect endocrine function in humans and wildlife should be investigated.imagesfigure 2figure 3figure 4figure 5figure 6