Sources variability with Planck LFI

Planck LFI (Low Frequency Instrument) will produce a complete survey of the sky at millimeter wavelengths. Data stream analysis will provide the possibility to reveal unexpected millimeter sources and to study their flux evolution in time at different frequencies. We describe here the main implications and discuss data analysis methods. Planck sensitivities typical for this kind of detection are taken into account. We present also preliminary results of our simulation activity.