  We report observation of oscillations in the dynamics of a microcavity
polariton condensate formed under pulsed non resonant excitation. While
oscillations in a condensate have always been attributed to Josephson
mechanisms due to a chemical potential unbalance, here we show that under some
localisation conditions of the condensate, they may arise from relaxation
oscillations, a pervasive classical dynamics that repeatedly provokes the
sudden decay of a reservoir, shutting off relaxation as the reservoir is
replenished. Using non-resonant excitation, it is thus possible to obtain
condensate injection pulses with a record frequency of 0:1 THz.
