The estrogen receptor (ER), a member of the nuclear hormone receptor superfamily recruits coactivators that modify local chromatin structure. Here we investigated the effect of the estrogen receptor and estrogen on the global chromatin structure and the local chromatin structure of the progesterone receptor gene during the process of transcriptional activation using Rat1+ER cells stably expressing the estrogen receptor. The total chromatin was more accessible to DNasel in Rat1+ER cells than in the parental estrogen receptor-negative Rat1 cells. After 18 h of estrogen treatment, total chromatin was more dispersed in Rat1+ER cells than in Rat1 cells. The chromatin structure of the progesterone receptor gene was more sensitive to DNasel in Rat1+ER cells than in Rat1 cells. However, the chromatin structure of the progesterone receptor gene did not change further on estrogen treatment. Our results suggest that under certain circumstances unoccupied estrogen receptors may play some role in reorganizing the repressive chromatin structure to induce gene activation.