Immunocytochemical methods were used to define the distribution of enkephalin (ENK), substance P (SP), tyrosine hydroxylase (TH), and serotonin (5-hydroxytryptamine: 5HT) in the rat septum. A dense plexus of axons containing enkephalin-like immunoreactivity is found in the intermediate lateral septal nucleus. This is surrounded laterally by SP-containing cell bodies and axons and medially by ENK-containing cell bodies. Both SP- and ENK-immunoreactive axons form pericellular and peridendritic terminal arbors around lateral septal neurons. TH-positive axons are distributed throughout the septum and form dense pericellular terminal baskets around scattered neurons in the medial half of the intermediate lateral septal nucleus and in the extreme lateral septum. Very few SP and TH immunoreactive axons are present in the ENK immunoreactive plexus zone. 5HT-immunoreactive axons are most dense at the lateral edge of the ventral and intermediate lateral septal nuclei but form pericellular terminal arbors only in the dorsal lateral septal nucleus, in the septofimbrial nucleus, and in the dorsal cap of the medial septal nucleus. These results indicate that the dorsal and intermediate lateral septal nuclei include three histochemically distinct laminated subfields: (1) an ENK immunoreactive axonal plexus within the lateral aspect of the intermediate lateral septal nucleus, (2) a more medial region of scattered ENK immunoreactive perikarya and similarly scattered TH immunoreactive pericellular baskets, and (3) a dorsolateral zone occupied by SP neurons and 5HT-containing pericellular baskets. Thus, the data suggest that SP- and ENK-containing neuronal populations in the lateral septum receive different monoaminergic inputs. Further, the somewhat exclusive laminated pericellular termination of peptide- and catecholamine-containing axons in the lateral septum predicts very different functional and pharmacological properties among zones.