Lesion-induced degeneration was combined with the retrograde transport of horseradish peroxidase (HRP) to examine the thalamocortical and other synaptical relationships of corticothalamic projection cells in mouse SmI cortex. Injections containing 40% HRP were placed in the ventrobasal thalamus and the next day, electrolytic lesions were made of the injection site. About four days later, the animals were perfused with aldehydes and SmI cortex ipsilateral to the injection and lesion sites was tissue chopped and reacted for HRP. The somata of HRP-filled corticothalamic cells in SmI cortex had diameters of about 10 micrometers and occurred in the upper half of layer VI and in the lower half of layer V; their apical dendrites usually terminated within or just below layer IV, but in some instances, extended nearly to the pial surface. Reconstructions of serial thin sections through the layer IV portions of the apical dendrites of seven well-labelled corticothalamic cells showed them to form 7 to 20% of their synapses with degenerating thalamocortical axon terminals. In contrast, the proportion of thalamocortical synapses formed by the layer IV dendrites of corticostriatal (Hersch & White, in preparation) and corticocortical (White & Hersch, 1981) projection cells ranges from 0.3 to 0.9% and from 1.5 to 6.8%, respectively. We interpret these findings to indicate that pyramidal cell dendrites are specified to form characteristic proportions of thalamocortical synapses.