To determine which ovarian hormone is involved in the sexually dimorphic antidiuretic action of vasopressin, the antidiuretic response to vasopressin was examined in sham-operated nonestrous female rats chronically treated with vehicle and in ovariectomized rats treated with vehicle, progesterone, estradiol, or the combination of estradiol and progesterone, respectively. Three-week-old female rats were sham operated or ovariectomized, and a slow-release hormone pellet was implanted at the 6th wk. The experiment was performed at the 10th to 12th wk in conscious, chronically instrumented rats hydrated with tap water (2% body wt). Infusion of vasopressin at rates of 10-1,000 pg.min-1.kg body wt-1 resulted in a dose-dependent antidiuretic response that was significantly enhanced in ovariectomized rats compared with the intact nonestrous females. Progesterone had no effect, whereas estradiol attenuated and restored the antidiuretic response to vasopressin to a level similar to that in intact nonestrous female rats. These results suggest that it is estrogen, but not progesterone, that reduces the antidiuretic response to vasopressin in the female rat.