Differential leucocyte population was studied at 0600h, 1200h, 1800h and 2400h, of a 24-hour period in the adult male parakeet. It was found that heterophil and eosinophil counts were higher at 0600h than at 2400h, while lymphocyte and basophil populations were less at 0600h than at 2400h. The finding indicates that there is a diurnal rhythm in the leucocyte population in the parakeet. This variation may be related to the activity cycle that varies between day and night in the bird.