The ipsilateral cortico-cortical connexions passing between the architectonic subdivisions of the primary somatic sensory cortex, S1, of the monkey have been studied with axonal degeneration methods after the placement of small lesions. All architectonic subdivisions except area 3a, and all the topographic representations, have been involved by the lesions. The degeneration of local intracortical fibres has the same features that have been described in other cortical areas: dense terminal degeneration for about 200 micron immediately around the lesion and moderate degeneration extending for a few millimetres with that in layers I, IV and the deep part of V being the most marked and reaching furthest; the degeneration extends further in the antero-posterior than in the medio-lateral dimension, and further posteriorly than anteriorly. The arrangement of the intercortical fibre connexions varies with the architectonic subdivision and with the topographic representation, and as in other sensory areas these fibres may be considered as either feed-forward or feed-back. The feed-forward projections are heavy, terminate in all layers of the cortex but mainly in layer IV and the deep part of layer III, whereas the feed-back connexions are lighter and end in layers I, II, the superficial part of layer III and in layers V and VI. In the antero-posterior dimension, feed-forward fibres from area 3b go to areas 3a, 1 and 2; area 1 sends feed-forward connexions to areas 3a and 2 and feed-back to area 3b; area 2 sends a feed-forward projection to area 3a and feed-back to areas 3b and 1; all areas also send fibres to area 5. A lesion in one of the architectonic subdivisions in the trunk and face representations results in degeneration throughout the antero-posterior extent of S1, but after damage within an architectonic area in the distal limb regions, there are foci of degeneration in the middle of the antero-posterior extents of the other areas but with little or none at the boundaries. The cortico-cortical fibres also extend medially or laterally for a few millimetres, in bands a few hundred micron wide. After damage of the trunk, occiput or proximal limb representations, the degenerating fibres pass predominantly along the boundaries; the separate representations of the caudal trunk, at the postcentral dimple and cingulate sulcus, are connected by continuous bands along the boundaries of area 3a and at the 2/5 boundary, and those of the occiput region at the levels of the postcentral dimple and lower end of the intraparietal sulcus are similarly linked.(ABSTRACT TRUNCATED AT 400 WORDS)