Mixed exciton–photon states in planar semiconductor microcavities with quantum wells in the active layer belong to a new class of quasi two dimensional states with unique properties. They arise due to a strong coupling of excitons with eigenmodes of electromagnetic radiation of a microcavity, as a result of which upper and lower exciton–polariton microcavity modes are formed. Large interest is drawn to polariton–polariton scattering, due to which the exciton–polariton system demonstrates strongly nonlinear properties. These nonlinearities were revealed in luminescence spectra of microcavities upon resonant excitation of the lower polariton branch, and they are explained by parametric scattering of photoexcited pump polaritons into signal and idle modes. The objective of this work is to study exciton–polariton dynamics in the parametric oscillator regime. It was shown in [1, 2] that, upon excitation of exciton–polaritons on the lower branch of the dispersion law, a parametric scattering of two pump polaritons (p) into polaritons of signal (s) and idle (i) modes, which are described by the interaction Hamiltonian of the form = ( + + + + + ) p p s i s i p p H aˆ aˆ aˆ aˆ aˆ aˆ aˆ aˆ int μ ,where μ is the constant of the parametric polariton–polariton conversion and p aˆ , s aˆ , i aˆ are the annihilation operators of polaritons of the corresponding modes (p, s, i). By using Hamiltonian, we obtain a system of Heisenberg equations for the operators ap, as, and ai. After averaging this system of equations and using the mean field approximation, we can obtain a system of nonlinear equations for the amplitudes of polaritons. In a case where the initial phase difference is equal to zero the periodic and aperiodic regimes are observed, as well as at rest the time evolution of the density of polaritons. The time evolution of the density of polaritons can occur in the aperiodic regimes when the normalized resonance detuning equal one in this case, all polaritons of the signal and idler modes transform in pairs into pump polaritons, after which the evolution is completed. When the initial phase difference is equal to π, as in the case the initial phase difference is equal to zero, there exist the periodic and aperiodic regimes of the evolution of the density of pump polaritons, as well as the vanishing of the amplitude of the periodic oscillations. The difference consists in the fact that, at the initial phase difference is equal to π, the aperiodic evolution occurs for different values of the resonance detuning, depending on the parameter ,whereas at the initial phase difference is equal to zero, this bifurcation takes place only for the normalized resonance detuning is equal to one. We found that, in the regime of a parametric oscillator, the dynamics of polaritons is a periodic conversion of a pair of pump polaritons into polaritons of the signal and idle modes and vice versa. The period and amplitude of these oscillations significantly depend on the initial polariton density, the initial phase difference, and the resonance detuning. At a certain relation between the parameters, the evolution of the system can also be aperiodic, as a result of which a part of pump polaritons convert into polaritons of the signal and idle modes, thus completing the evolution. The significant dependences of the period and amplitude of oscillations on the initial phase difference indicate that it is possible to perform the phase control of the dynamics of the system. A similar effect was previously predicted for the process of atomic–molecular conversion under the conditions of the Bose–Einstein condensation of atoms and molecules. [1] M. M. Glazov and K. V. Kavokin, Phys. Rev. B 73, 245317 (2006). [2] I. A. Shelykh, R. Johne, D. D. Solnyshkov, et al., Phys. Rev. B 76, 155308 (2007).