The effect of increasing serum estradiol levels on platelet [³H]PK 11195 binding was investigated to determine the influence of gonadal steroids on platelet peripheral benzodiapezine binding sites. Serum estradiol and platelet [³H]PK 11195 binding were determined in 10 anovulatory, but otherwise healthy women before and after treatment with human menopausal gonadotropin. Despite a 30-fold increase in posttreatment estradiol, the kinetic parameters of platelet [³H]PK 11195 binding (K_d, B_max) remained essentially unchanged. These data indicate that peripheral benzodiazepine binding sites on platelets are not influenced by elevated concentrations of estradiol. Thus, it is suggested that variations in circulating levels of this female gonadal steroid are likely not responsible for the modifications in platelet peripheral benzodiazepine binding sites seen in association with various disease states.