  The tachyonic regime of the quantum fluctuations of a self-interacting scalar
field around its vacuum mean value is studied within a kinetic approach. We
derive a quantum kinetic equation which determines the time evolution of the
momentum distribution function of produced tachyonic modes and includes memory
effects. The back-reaction of the quantum fluctuations on the vaccum mean field
is taken into account, while their interaction is neglected. We show that the
tachyonic modes do not correspond to real particles and contribute to the decay
of the metastable vacuum state.
