Immunahistochemical double staining for gonadotropin releasing hormone (GnRH) and serotonin or dopamine-β-hydroxylase reveals close appositions of fibers which contain serotonin or norepinephrine to GnRH producing neurons in the septo-preoptic region. In the organum vasculosum of the lamina terminalis and in the median eminence extensive anatomical overlap exists in the distribution of GnRH and serotoninergic fibers but little of GnRH and noradrenalinergic fibers. It is proposed that serotonin plays a major role in the regulation of GnRH secretion via contacts in all of the regions studied and that the influence of norepinephrine on GnRH-secretion in the median eminence is exerted mainly via involvement of dopaminergic tuberoinfundibular neurons.