Visual experience plays a critical role for the formation of functional maps in the mammalian visual cortex. Here we used two-photon Ca2+ imaging to study the development of visually-evoked Ca2+ signals in the mouse primary visual cortex. Even before eye opening (from postnatal day 11 (P11) on) brief light flashes, when presented at a distance of 30 cm from the contralateral eye, effectively evoked Ca2+ transients in the majority of the recorded neurons. At earlier ages (P8-P10) the activation of the cells was possible only if the stimulus was presented directly in front of the eye. These early responses were characterized by rather long latencies ([fnof]î400 ms). The latencies continuously decreased with development reaching 80 ms at eye-opening (P14) and 60 ms at P40. The fraction of responding cells was highest (90%) at the youngest ages and continuously decreased with development. During the first three postnatal weeks, neurons poorly responded to moving gratings. The orientation selectivity improved during the fourth postnatal week. Around P30 moving gratings evoked large Ca2+ transients in about one third of the neurons tested. Neurons with different orientation selectivity were randomly distributed in the field of view. Thus, during the first week after eye-opening (= precritical period) the responsiveness to defined visual stimuli is poor or even absent, but it substantially improves during the subsequent stage known as the critical period for monocular deprivation.