The involvement of the nucleus intercollicularis (ICo) region of the midbrain, an estrogen-sensitive area, in the expression of estrogen-dependent female courtship behavior was examined with radiofrequency lesion and intracranial hormone implant techniques. Bilateral lesions in the ICo region caused a reduction in the female's nest coos, and no reduction in other behaviors in response to male courtship. In addition, the follicles of the ICo lesioned females failed to grow in response to male courtship. A second experiment showed that the difference in follicular size between ICo-lesioned and sham-ICo-lesioned females was not observed if females were visually isolated from males, suggesting that tonic ovarian activity was not affected by ICo lesions. In the third experiment, unilateral 30-gauge implant containing estrogen (E) or estradiol benzoate (EB) in the ICo region of bilaterally ovariectomized females restored the nest-coo, though not to the level obtained with systemic EB injections. Other behaviors were not selectively affected by E or EB implants in the ICo region. Diffusion from the implant was probably not responsible for the elicitation of the nest-coo, since the oviduct weights of females implanted with E or EB and cholesterol did not differ. These data are consistent with the hypothesis that the ICo region is involved in the expression of vocal (cooing) courtship behavior in the female ring dove.