In the normal guinea pig a map of auditory space appears, in the deeper layers of the superior colliculus, at 32 days after birth (DAB). The animal is unable to construct this collicular map of auditory space in the absence of developmental visual experience. Auditory receptive fields of animals dark-reared from birth are typically large, occupying most of the contralateral hemifield. There is no topographic relationship between the collicular location of the recording electrode and the spatial position from which auditory stimuli elicit a maximal response. The fields of dark-reared animals resemble, in their tuning parameters, the spatially undifferentiated fields typical of young postnatal normal guinea pigs. To investigate the time-course during which visual experience is required for map emergence, animals received normal visual experience until either 18 or 26 DAB and were then dark-reared until the terminal mapping experiment. Maps developed in neither group. Animals provided with a normal visual environment until 30 DAB, and then placed in the dark did, however, construct topographically organized spatial maps with discrete spatial receptive fields. Maps also failed to emerge in animals receiving normal visual experience both before and after a 4-day period of visual deprivation between 26 and 30 DAB. We conclude that this 4-day period, or part of it, constitutes a 'crucial' period during which visual experience is required for the normal elaboration of the collicular map of auditory space.