Innervation of the spleen in rats was studied. Butyrylcholinesterase-(BuChE)-positive nerve components of the organ were visualized by the direct thiocholine method. BuChE-positive nerve components enter the spleen in a common bundle with arteries. In the organ they form characteristic periarterial and periarteriolar plexiform arrangements, which are especially conspicuous around the aa. centrales running through the white pulp. Then, nerve fibres extend away from these plexuses into adjacent layers of trabeculae further into marginal layers of periarterial lymphatic sheath (PALS) as well as into the mantle zone of follicles. Several scattered periarteriolar and solitary nerve fibres can be seen in the marginal sinuses and cords of the red pulp. In the fibrous capsula BuChE-positive nerve fibres can also be seen which have an evident connection with trabecular and parenchymal nerves of the organ. Microscopic findings support the notion that BuChE-positive nerve profiles supply not only the vasculature, but also the parenchymal components of the spleen, and they may participate, to a great extent, in the regulation of the immune processes in this organ.