Congenitally hypogonadal (hpg) male mice are unable to synthesize biologically active gonadotropin-re-leasing hormone (GnRH). Implantation of normal fetal preoptic area tissue containing GnRH neurons into the third ventricle of adult hpg males significantly elevates pituitary levels of luteinizing hormone (LH) and corrects their hypogonadism. In all responding animals, immunoreactive GnRH neurons within the transplant innervate the median eminence of the host. To assess whether gonadal recovery in hpg hosts results from pulsatile secretion of GnRH from grafted neurons, we compared the pattern of variation in plasma LH levels in 19 hpg graft recipients with testicular growth to that of 10 normal adult mice. All animals were castrated prior to receiving an indwelling catheter in the jugular vein. Sequential blood samples were collected (t = 10 min) and assayed for LH. Pulsatile LH secretion was seen in 11 of 19 hpg hosts and in all control mice. While there was great variability between individual animals, measures of baseline LH, LH pulse amplitude and duration, interpulse interval, and LH pulse frequency revealed no difference between hpg graft recipients and normal castrates in their LH pulse pattern. Immunocytochemical analysis of the brain in hpg hosts suggested no correlation between any parameter of pulse activity and individual differences in GnRH cell number or GnRH fiber outgrowth into the median eminence. Sources of variation in LH secretion among graft recipients, and between hpg hosts and normal mice, are discussed. We suggest that transplanted GnRH neurons are capable of integration into a GnRH ‘pulse generator’ which can support a near-normal pattern of pulsatile LH secretion, leading to testicular growth and steroid product