Detailed histochemical studies have been performed on the distribution of the nuclei innervating the supraoptic nucleus (SO) in the whole brain after horseradish peroxidase (HRP) injections through fine glass micropipettes (2 to 8 micrograms ) into the SO of 39 Wistar strain rats. The chromatogenic reaction was carried out according to the Mesulam's (1976) No. 8 procedure. Successful HRP injections resulted in labeling in various regions of the brain. The following conclusions were drawn from the analysis of these successful cases. All the nuclei of the subthalamus and posthypothalamus innervate the SO whereas only a few neurons of the hypothalamic arcuate and ventromedial nuclei probably send axons to it. The afferent monoaminergic fibers arise largely from the B7 and B8, and to a certain extent from the A6 and A10 cell groups. The septum, diagonal band nucleus and amygdala project directly to the SO while the olfactory tubercle and pyriform cortex may not. The mesencephalic central gray has a high possibility to project directly to it. The possibility that the hippocampus sends axons to the SO remains to be fully proven. A few small multipolar cells clearly revealed by the test in the SO should be non-secretory intercalated neurons.