Sulfates are the major circulating conjugates of ethinyl estradiol and glucuronides predominate in urine. The primary oxidative metabolite is 2-hydroxy ethinyl estradiol, formed through the CYP3A4 isoform of cytochrome P450. Part of the initial-pass metabolism of ethinyl estradiol is considered to arise in gastrointestinal mucosa. Ethinyl estradiol