The haematology and serum chemistry parameters in the rat as a function of age were determined. Age had a significant effect on many of the parameters studied. Albumin, cholesterol, triglycerides, urea, albumin:globulin ratio, alkaline phosphatase, aspartate aminotransferase, glutamic pyruvate transaminase and lymphocytes (%) were found to decrease with age, while an age-associated increase was detected in blood glucose, total proteins, creatinine, red blood corpuscles, haemoglobin and haematocrit value. The physiological basis of these age-related animal model which does not gain weight and become obese with age, and avoiding aging studies which are based on comparison between young and senescent animals.