Microcavity polariton spin dynamics is investigated in high-finesse microcavities under resonant excitation using polarization- and time-resolved secondary emission spectroscopy. Using an appropriate calibration procedure, the instantaneous polariton occupation numbers of the modes with k 0 wave vectors can be determined. When the phase-matching conditions are achieved, we show that Coulomb scattering dominates the other processes, and that stimulation occurs when the occupation of the k 0 final state with a given mode polarization approaches unity. Under elliptically polarized light, we show that the stimulated polariton scattering leads to an increase with time of the circular polarization of the k 0 states, while the linear polarization dynamics is characterized first by a fast drop to negative or zero values immediately after the excitation pulse, followed by weak-amplitude nonperiodic oscillations. The latter are interpreted as the manifestation of the pseudospin precession of the pumped polariton modes around the self-induced exchange effective field.