AbstractPyramidal neurons in the mouse SmI cortex were labeled by the retrograde transport of horseradish peroxidase (HRP) injected into the ipsilateral MsI cortex. Terminals of the local axon collaterals of these neurons (CC terminals) were identified in SmI, and their distribution and synaptic connectivity were examined. To avoid confusion, terminals in SmI cortex labeled by the anterograde transport of HRP injected into MsI were eliminated by lesioninduced degeneration. Lesions of MsI were made 24 hours after the injection of HRP; postlesion survival time was 4 days.Most CC axon terminals occurred in layers III and V where they formed asymmetrical synapses. Of 139 CC synapses in layer III and 104 in layer V, approximately 13% were formed with dendritic shafts. Reconstruction of 19 of these dendrites from serial thin sections showed them to originate from both spiny and nonspiny neurons. Most synapses of CC terminals (about 87%) were onto dendritic spines. In contrast, White and Keller (1987) demonstrated that terminals belonging to the local axon collaterals of corticothalamic (CT) projection cells synapse mainly with dendritic shafts of nonspiny neurons: 92% onto shafts, the remainder onto spines. The distribution of asymmetical synapses onto spines and dendritic shafts was analyzed for neuropil in layers III, IV, and V. Depending on the layer, from 34 to 46% of the asymmetrical synapses in the neuropil were onto dendritic shafts. Results showing that CC and CT terminals form proportions of axodendritic vs. axospinous synapses that differ from each other, and from the neuropil indicate that local axon collaterals are highly selective with regard to their postsynaptic elements.