This study evaluated the haematology and clinical chemistry profile of rats given drinking water contaminated with varied low percentages of used engine oil (UEO) for a period of 21 days. Fifty female albino rats of 6–7 weeks of age were used for the study. They were divided into five groups (A–E) and given water contaminated with 5%, 1%, 0.1%, 0.01% and 0% vol/vol. of UEO respectively as the only source of drinking water for 21 days. The group E given uncontaminated water (0% contamination) served as the control. The haematological parameters and clinical chemistry profile of the rats was comprehensively evaluated after the 21 days of administration of the group-specific waters. Results showed that contamination of water with up to 5% UEO led to no significant effects (p > 0.05) on all the haematological indices and on the levels of serum alanine amino transferase, aspartate amino transferase, albumin, creatinine and calcium, blood urea nitrogen and fasting blood glucose level, feed consumption and body weight. However, the rat group given water contaminated with 5% UEO had a significantly increased serum alkaline phosphatase (AP) (p < 0.01), total bilirubin (p < 0.05) and cholesterol (p < 0.01), and a significantly decreased serum total protein and globulin (p < 0.01), and water consumption (p < 0.05). The rat group given water contaminated with 1% UEO had a significantly increased serum AP (p < 0.01), total bilirubin (p < 0.05) and cholesterol (p < 0.01), and a significantly decreased water consumption (p < 0.05), while the rat group given water contaminated with 0.1% UEO had a significantly elevated (p < 0.01) serum AP. It was concluded that sub-acute contamination of drinking water of rats with up to 5% UEO led to hepato-biliary disorders and adverse effects on hepatic secretion and excretion, including diminution of serum protein and globulin levels and elevation of serum cholesterol levels, but did not lead to any significant effects on haematology, hepatocellular integrity, kidney/renal function, pancreatic function and body weight.