Positron emission tomography measurements of regional cerebral blood flow were used to detect focal neuronal activation in the first somatosensory cortex (SI) of humans induced by cutaneous vibratory stimulation. Intravenously administered water labeled with oxygen-15 (H2(15)O) was used as a blood flow tracer to obtain five stimulated-state and two resting-state blood flow images in each of eight normal volunteers. Three cutaneous surfaces were tested: lips, fingers, and toes. Intense, highly focal SI responses were seen during all 39 stimulated-state trials. The SI responses from the three stimulation sites were anatomically distinct and formed a medial-to-lateral homonculus in every subject. Response magnitudes (increase in local blood flow) and response locales (expressed as proportionately measured bicommissural stereotaxic coordinates) were highly consistent among subjects and on repeated trials for each subject. These findings suggest that eliciting cerebral blood flow responses by cutaneous vibration provides a safe, rapid, and reproducible tool for locating and assessing the functional status of somatosensory cortex, and offers potential clinical and research utility. This study has established normative values for future applications of this experimental paradigm.