MCF-7 breast tumor cells form multicellular foci in vitro when supplemented with 17 beta-estradiol (E2). In the presence of E2 and the aryl hydrocarbon-receptor agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), MCF-7 cells grow to confluence but do not form foci. To investigate the role of E2 metabolism in this antiestrogenic effect of TCDD, analyses were performed by capillary GC/MS. The results revealed that pretreatment of MCF-7 cultures with TCDD (10 nM) rapidly depletes E2. In untreated cultures supplemented with 10 nM E2, the concentration of free E2 decreased to 4 nM in the first 12 hr, followed by a slower rate of decline. After 3 days most E2 in the medium was in conjugated form(s); 1.7 nM was present as free E2, and 2.9 nM was released by treatment with glucuronidase/sulfatase. In TCDD-treated cultures, E2 declined to 290 pM in 12 hr and after 2 days was not detected (less than 100 pM) either as free steroid or after treatment with glucuronidase/sulfatase. Intracellular E2 and estrone were likewise depleted by pretreatment with TCDD. Microsomes from TCDD-treated cells showed highly elevated aryl hydrocarbon-hydroxylase activity and catalyzed hydroxylations of E2 at C-2, C-4, C-15 alpha, and C-6 alpha with a combined rate of 0.85 nmol/min per nmol of cytochrome P-450 at saturating E2. These results suggest that depletion of E2 by enhanced metabolism accounts for the antiestrogenic activity of TCDD in MCF-7 cells.