We tested the hypothesis that somatosensory discrimination of roughness (microgeometry) but not of shape (macrogeometry) would activate the parietal operculum (PO) in man. It was also investigated whether a simple square pulse indentation of the skin on the index finger would activate the PO. Regional cerebral blood flow was measured with [15O]butanol and positron emission tomography in a total of 20 normal volunteers. Ten subjects used their right hand to discriminate objects that differed in roughness and similar smooth objects that differed in length. Ten other subjects pressed a button when they felt a square pulse indentation of the skin on their right index finger in a somatosensory reaction time task. Discrimination of roughness activated one field in the PO contralaterally and two fields ipsilaterally to the stimulated hand. The discrimination of length activated one field in the PO located ipsilaterally to the stimulated hand. The somatosensory reaction time task also activated one contralateral and two ipsilateral fields in the PO, and these fields partially overlapped the activated fields in the roughness discrimination task. Based on the extension of these fields and their overlaps we conclude that there exist at least one part of the contralateral PO and at least two parts of the ipsilateral PO that can be activated by somatosensory stimulation of the right hand. We argue further that the contralateral activated part contains a region than can be activated by roughness.