The diurnal rhythm of haematological parameters was investigated in the male African giant rat ( Cricetomys gambianus
, Water house). There was a significant difference (P<0.001) between the values of red blood cell count (RBC) obtained during the day (light phase) and at night (dark phase). The lowest value (RBC) was recorded at 12.00hrs, but showed a gradual increase from 12.00 hrs to 24.00hrs, although the differences were not significant. It reached a peak at 21.00 hrs. The packed cell volume (PCV) showed a progressive decline between 9.00 hrs and 24.00hrs. The value obtained at 24.00 hrs was also significantly lower than the values at 9.00 hrs (P<0.01) and 12.00 hrs (P<0.01). White blood cell count (WBC) gradually decreased during the period of the study. The WBC values obtained at 24.00 hrs were significantly lower than the values at 9.00 hrs (P<0.01) and 12:00 hrs (P<0.02).