In the mammalian cortex the critical period is a time window during which neural circuits are intensely shaped by experience. Using in vivo two-photon Ca2+ imaging and patch-clamp recordings we characterized activity patterns present in the mouse visual cortex during this time. Around eye opening many individual layer 2/3 neurons exhibited synchronized Ca2+ transients. These spontaneous Ca2+ waves were first observed at postnatal day 8 (P8). Their frequency continuously increased from 0.2 waves/min at P8 to 40 waves/min at P60. The membrane potential in these cells oscillated between two discrete levels reminiscent to the well known up and down states. The up states were often accompanied by bursts of action potentials. Initially the network activity was rather profuse involving 60-90% of the neurons. The fraction of active cells dropped considerably during the third postnatal week to 20% of active cells/wave. This decrease was paralleled by a decrease in the fraction of suprathreshold up states. The switch in the activity pattern correlated with the beginning of the critical period at P21 and was delayed by dark rearing of the mice. Thus, the onset of the critical period in the mouse visual cortex is linked to a developmental switch in the pattern of endogenous activity transforming a global, profuse activation of the neural network into a sparse one. This transformation might reflect the experience-dependent specification of neural elements in the visual cortex circuitry.