The hypogonadal mouse is one of "nature's knockouts," bearing a specific deletion in the gene for gonadotropin-releasing hormone (GnRH), with the result that no GnRH peptide is detectable in the brain. The lack of reproductive development after birth provides an animal model that has proved fruitful in clarifying the role of GnRH in reproductive behavior and physiology. Behavioral studies with hypogonadal mice convincingly demonstrate that although GnRH may facilitate the appearance of sexual behavior, this peptide is not essential for either male or female sexual behavior in the mouse. Administration of GnRH to hypogonadal mice with regimens mimicking GnRH pulsatility initiates reproductive development. Surprisingly, continuous exposure to GnRH stimulates remarkable ovarian and uterine growth and increased FSH release, although pituitary content of LH and FSH remains unchanged. In contrast, when brain grafts of normal fetal preoptic area (POA), containing GnRH cells, are implanted in the third ventricle of adult hypogonadal mice, both pituitary and plasma gonadotropin levels increase. Grafted GnRH neurons innervate the median eminence of the host and support pulsatile LH secretion in the majority of animals with graft-associated gonadal development. Studies of hypogonadal mice with POA grafts demonstrate that distinct components of reproductive function are dissociable: hosts may demonstrate reflex but not spontaneous ovulation; others may show positive but not negative feedback. Activation of grafted GnRH cells in response to sensory input to the host, as revealed in Fos expression studies, is an example of the integration of the graft with the host brain that underlies such capabilities. A goal of these studies is to elucidate the specific connectivity underlying discrete aspects of reproductive function.