The effect of Bonny light crude oil on some haematological parameters (red cell count, white cell count, platelet count, packed cell volume and hemoglobin concentration) were studied in guinea pigs. The two experimental groups were gavaged with 4ml/kg and 6ml/kg respectively for seven days while the control group was gavaged with olive oil (6ml/kg) for seven days. The red cell count in the 4ml/Kg group was significantly lower (P<0.05) when compared with control and 6ml/Kg group. The platelet count showed statistically significant difference (P<0.05) when the two test groups were compared with the control and also when compared to each other (P<0.05). Haemoglobin concentrations were significantly increased (P<0.05) in high dose group compared with low dose group and control. There were no significant differences in haemoglobin concentrations of low dose group when compared with control. The results indicate that gavaging of guinea pigs with Bonny light crude oil cause changes in some hematological parameters due to the injurious effect of the toxic components of the crude oil.