An examination of material prepared for conventional electron microscopy has indicated that there are at least four different types of axon terminals forming symmetric synapses with the cell bodies and initial axon segments of layer II/III pyramidal cells in the rat visual cortex. One type of terminal synapses with the initial axon segment and it is derived from the chandelier cell. Because the location and features of these terminals allow them to be readily recognized, chandelier cell terminals were used to determine the extent of morphometric variability that can exist among terminals originating from one cell type. It was found that there is a wide range of mean synaptic vesicle size among chandelier terminals, so that calculated mean vesicle profile diameters for individual terminals can be between 32 and 39 nm. Similar ranges of mean synaptic vesicle sizes also exist among populations of the other three axon terminal types. These terminal types are referred to as 'large', 'medium-sized', and 'dense' terminals. The large terminals synapse with the cell bodies of layer II/III pyramids and their profiles often measure 1.5 X 0.8 microns. The large terminals contain rather loosely packed pleomorphic vesicles and they frequently synapse with a second neuronal element. The medium-sized terminals are smaller, being 1.0 X 0.6-0.8 microns in size, and their synaptic vesicles are usually more closely packed than those within the large terminals. The medium-sized terminals are the ones encountered most frequently on the cell bodies of pyramidal cells and they can also occur on the axon hillock and initial axon segment. The dense terminals are usually flattened against the cell body, and they contain rather rounded and closely packed synaptic vesicles, which often seem to be enmeshed in a rather dark cytoplasmic matrix. This matrix and the close packing of the vesicles makes these terminals appear to be more dense than the others. It is now necessary to determine the origins of the large, medium and dense terminals, and to ascertain if they all use GABA as their neurotransmitter.