Recently, we reported a new method for the detection of calcium signals in anesthetized and awake animals using implanted optical fibers. With this approach we identified early network oscillations in the cortex of newborn mice 'in vivo' (Adelsberger et al., Nat. Neurosci. 2005). Here we report the presence of a new type of spontaneous calcium waves in the auditory cortex of older mice. These calcium waves occurred at postnatal day 6 and had initially a frequency of 0.013±0.002 Hz (n=5 mice). The frequency of the calcium waves increased during later stages of development and reached its maximum of 0.2±0.09 Hz (n=7) at postnatal day 20. The calcium waves persist at this frequency in adult mice. In the presence of anesthetics (e.g. isoflurane), the frequency of the calcium waves dropped about 5-6 times. Besides these spontaneous oscillatory waves, also sound evoked calcium waves could be detected from postnatal day 12 on. Already at the onset of hearing, a tonotopic organization of the primary auditory cortex was observed. The delays of sound evoked signals decreased from 34±0.0008 ms at the onset of hearing to about 20 ms at postnatal day 30. Furthermore, in awake animals at ages of 12 to 13 days the threshold level was higher and the decay time constants of evoked transients were remarkably longer. Taken together, our results indicate that the calcium signals represent an integral part of the primary auditory response in the cortex.