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Intercortical connections between different groups of neurons, the so-called "barrels" located in the somatic sensory cortex of the rat brain were identified by the retrograde horseradish transport technique. It is shown that HRP-labelled cells were found at a distance up to 400 microns from the place of injection. 90% of them are pyramidal neurons located mainly in laminae V and III of the cortex. The number of intercortical connections in labelled neurons was 1.6 times as high as that of thalamocortical ones. It is supposed that interaction between neurons of neighbouring groups is realized by the pyramidal cells of laminae V and III.

Localization and morphology of neuronal groups in the ventrobasal thalamic complex that project to the vibrissae area in the somatosensory rat cortex were studied by means of retrograde axonal transport of horseradish peroxidase (HRP). The HRP-labelled cells were located in the ventroposterolateral nucleus of the ventrobasal complex and were grouped in "barreloids" 100-120 microns wide. Staining of several "barreloids" even when the injection was made in a restricted area of the cortex and the growth of labelled cells number in each "barreloid" with an increase in the injection area suggested the existence of divergence as well as convergence of thalamocortical specific connections. Various forms of relay neurons and different extent of HRP-granule inclusion may reflect their different functional value in thalamocortical relationship.