Administration of o,p'-DDT to immature female rats causes a translocation of estrogen receptors to uterine nuclei which is maximum 3 hours after pesticide treatment, and a subsequent increase in uterine weight at 24 hours. A high degree of correlation (r = 0.98) exists between the level of nuclear estrogen receptor and increases in uterine weight measured 3 and 24 hours respectively after administration of o,p'-DDT, in the dose range of 10-1000 mg/kg. This high degree of correlation between these two parameters lends strong support to the hypothesis that the estrogenic effects observed following in vivo administration of o,p'-DDT result from the interaction of the pesticide with the classical estrogen receptor system.