The somatosensory map of the superior colliculus was studied in the mouse during the 2nd and 3rd postnatal weeks, with particular attention to the representation of the vibrissae. The presence of an organized somatosensory projection was not electrophysiologically demonstrable until the beginning of the 3rd postnatal week, in coincidence with the opening of the eyes. Recordings performed during the 2nd postnatal week showed that the collicular units responding to tactile stimuli were rare and functionally immature. Starting from the beginning of the 3rd postnatal week, there was a dramatic increase of the number of somatosensory units and the appearance of an ordered topography, together with the maturation of the neuronal electrical properties. All the features of the adult collicular somatosensory map were found in the 3rd postnatal week, including the magnification of the vibrissae occupying the central portion of the visual field. The functional maturation of the collicular somatosensory map of the mouse is therefore a postnatal event which starts during the 3rd postnatal week, just after the opening of the eyes.