The estrogenic activity of DDT and some of its homologues was evaluated in female rats by using three experimental models: 1) young rats treated for 27 days, 2) mature ovariectomized rats treated for one week, and 3) mature hemiovarjectomized rats treated for one week. In the first model, 500 μg o,p′-DDT injected daily produced advanced vaginal opening and heavier ovaries and uteri. Utilizing the second model, 10.0 mg/day of o,p′-DDT and p,p′-DDA produced increased uterine wt and a thickened, vacuolated uterine epithelium. The former compound also induced cornified vaginal smears and reduced serum LH concentrations without significantly affecting FSH titers. Neither compound affected the compensatory hypertrophy of the remaining ovary in the hemicastrated rat model. The p,p′-DDD and p,p′-DDE homologues failed to show estrogenic activity. These results demonstrate that o,p′-DDT and one of its major metabolites, DDA, exert estrogenic activity by stimulating uterine and vaginal tissue and reduce serum LH, probably via an inhibitory feedback action at the pituitary or hypothalamic level. (Endocrinology91: 1095, 1972)