  We present the first fully non-linear calculation of inflaton decay. We map
inflaton decay onto an equivalent classical problem and solve the latter
numerically. In the $\lambda\phi^4$ model, we find that parametric resonance
develops slower and ends at smaller values of fluctuating fields, as compared
to estimates existing in literature. We also observe a number of qualitatively
new phenomena, including a stage of semiclassical thermalization, during which
the decay of inflaton is essentially as effective as during the resonance
stage.
