The fine structure and types of contact made by GABAergic elements in the septal nuclei were studied at the electronmicroscopic level by means of peroxidase immunocytochemistry, using anti-GABA antibodies. Observations were made on normal and colchicine-injected rats. GABA-immunoreactivity was distributed within somata, dendrites, axonal varicosities and terminals, and myelinated axons. The peroxidase reaction product was diffuse in the cytoplasm; cytoplasmic organelles were generally devoid of immunoreactivity, while showing a strong reaction on the outer surface of their membrane. GABA-immunoreactive (GABA-I) neurons were small (10 microns on average) to medium (20 microns) in size, with round or multipolar cell bodies. Additionally, labeled large (30 microns) cells were observed within the myelinated fibers of the medial septal nucleus after intraseptal administration of colchicine. No difference in the ultrastructural features and distribution of the immunoreactivity of the 2 kinds of cell was noticed, except for a higher number of synaptic contacts on large neurons of the medial septum. GABA-I cells of the medial and lateral nuclei received synapses on their soma and dendrites, made by both immunonegative and GABA-I terminals. Nonimmunoreactive boutons contacting GABA-I cell bodies were of 2 types: those containing small, clear synaptic vesicles and those that additionally contained large dense vesicles. Synaptic vesicles of GABA-I boutons were rarely labeled internally, but showed varying electron densities. Synapses made by GABA-I boutons on GABA-I or unlabeled somata and dendrites were always of symmetrical type. Synapses made by non-GABA-I boutons on GABA-I cells were either symmetrical or asymmetrical.