The distribution relationships of gonadotropin-releasing hormone (GnRH) (5-HT), tyrosine hydroxylase (TH), neurophysin (NP) and ?-endorphin (?-END) in the diagonal band of Broca (DBB) of rat brain were studied by immunohis- tochemistry. The results indicate that there was a close spatial overlap in the distribution between 5HT,TH,NP,?-END fibers and GnRH perikarya and fibers in the ventromedial area of VDBB and dorsomedial area of HDBB. In the dorsomedial area of VDBB close overlap in the distribution between 5-HT fibers and GnRH neurons was also observed. In addition, some TH and NP immunoreactive neurons were found in the medial areas of VDBB and HDBB, The close relationships in distribution and functional significance of GnRH,5-HT, TH, NP and ?-END were discussed.