The effect of unilateral orchidectomy on plasma FSH, LH and LHRH content in the right and left half of the median eminence-arcuate nucleus region was examined in adult rats. Following right-sided orchidectomy, plasma FSH began to rise but a significant increase was not observed until 1 week after the operation. No significant changes were observed in LH values. LHRH content in the ipsilateral median eminence-arcuate region showed a significant increase only when the median eminence was removed together with the arcuate nucleus, but not when it was removed separately, which suggests that the increase in LHRH content was attributable to an increased LHRH content of the ipsilateral arcuate nucleus. Radiofrequency lesions placed unilaterally in the same side of the dorsal anterior hypothalamic area as the hemiorchidectomy partially blocked the selective FSH release following unilateral orchidectomy, while contralateral or sham lesions had no effect. These results suggest that the gonad has a direct neural connection with the hypothalamus, which is involved in selective FSH release following hemiorchidectomy.