- We have recently organized and directed a consortium of manufacturers to assemble a unique electron microscopic-video-computer resource that reduces the time and effort required to make 3-dimensional reconstructions of neurons, and to collect and analyze data on synaptic organization by many fold. With the introduction of this system the quantitative study of synapses has entered a new age, characterized by a markedly increased efficiency that allows previously unrealistic studies to be carried out. In this paper, we present data designed to test the hypothesis that pyramidal cell types, identified by their intrinsic firing patterns, display characteristic inhibitory responses and distinctive synaptic patterns. These studies focus on the synaptic connectivity of regular spiking (RS) and intrinsically bursting (IB) neurons. Previous studies have shown that these neurons display distinct differences in their intrinsic membrane properties and in their morphologies as assessed with the light microscope. Under various conditions the synaptic responses of RS cells display marked inhibitory postsynaptic potentials, whereas most IB cells do not. This investigation is designed to determine if differences in the inhibitory responses of RS vs. IB cells are reflected in differences in the concentration of inhibitory synapses onto their somata. RS and IB neurons in rat somatosensory cortex were identified using intracellular recording and labeling, examined with the light microscope, and then serial thin sectioned prior to examination with the electron microscope. Synapses onto their somata and proximal dendrites were identified and plotted onto computer-assisted 3-D reconstructions made from the serial thin sections. Our analysis showed no significant difference in the types and concentration of synapses made onto the cell bodies and proximal dendrites of IB vs. RS neurons. Thus the differences observed in the inhibitory responses of IB vs. RS neurons cannot be explained by differences in the concentrations of synapses onto their somata.