gas-1 belongs to a family of growth arrest specific genes negatively regulated after growth induction of arrested cells. We report the expression of gas-1 in an in vivo system of cell proliferation. gas-1 mRNA accumulates progressively in the uterus of ovariectomized rats with a peak at three weeks after surgery. After estrogen treatment gas-1 mRNA levels decrease within two hours, at a time when c-myc expression is greatly increased, and return to pretreatment levels at 48 hours. Treatment with cycloheximide does not prevent estrogen-induced down-regulation of gas-1 mRNA levels. The present results show that: i) estrogen affects the uterine growth state by regulating the expression of both positively and negatively acting genes, ii) gas-1 expression is controlled by cellular growth state also in vivo.