Estradiol was applied directly to the striatum of ovariectomized female rats by a unilateral intracerebral cannula for three hr or four days. Following four days of estradiol treatment, rats increased the number of rotations in the direction away from the side of the hormone treatment. Cholesterol-treated animals did not change their rotational behavior. Dopamine receptors were assayed in the same animals by autoradiography; D2 receptors increased on the hormone-treated side relative to the untreated side after four days of treatment, only in the lateral striatum. D1 dopamine receptors did not change. The D2:D1 receptor ratio was related to the direction of rotation. Measurements of membrane fluidity with a fluorescent probe revealed no effect of estradiol on striatal membrane fluidity. Membrane proteins were labeled with estrogen agonist and antagonist affinity labels and analyzed by gel electrophoresis, but no saturable membrane binding sites were detected. The results indicate that estradiol acts directly in the striatum to affect behavior and dopamine receptors, but the neurochemical mechanisms remain to be determined.