The temperature and carrier-density dependent excitonic relaxation in bulk ZnO is studied by means of time-resolved photoluminescence. A rate-equation model is used to analyze the population dynamics and the transitions between different exciton states. Intra-excitonic (n #x2009;= #x2009;1) to (n #x2009;= #x2009;2) relaxation is clearly identified at low excitation densities and lattice temperatures with a characteristic time constant of 6 #x2009; #xb1; #x2009;0.5 ps.