Norepinephrine (NE) and dopamine (DA) levels were measured in several estrogen concentrating brain regions over the estrous cycle and in steroid-primed ovariectomized rats under experimental conditions used to study sexual receptivity. Norepinephrine content in the ventromedial hypothalamus (VMH), lateral septum (LS) and medial preoptic area (POA) varied during the estrous cycle. The highest NE levels were found during metestrus in these brain regions. In the ovariectomized rat, NE levels were increased in the POA and LS 12 hours (hr) after the injection of 3 micrograms of estradiol benzoate (65% and 33%, respectively). Norepinephrine content in the POA and LS remained elevated 24 hr and 48 hr (0.5 mg progesterone at 42 hr) after treatment but were not different from control levels at 96 hr. Dopamine concentrations were not significantly altered over the estrous cycle or by the administration of estradiol benzoate in any of the brain regions studied. These results suggest that estrogen may selectively alter noradrenergic activity in brain regions that are implicated in the control of sexual receptivity and/or ovulation.