Twenty mature albino rats randomly assigned to two groups (n=10) were used for the study. Group A was exposed to the sun while group B was kept under a shade. The study was for 28 days. At the end of the study the effects of heat stress on haematology and serum biochemistry were determined. There was a significant increase (p > 0.05) in the packed cell volume of group A when compared to group B. Red blood cell counts of group A increased significantly when compared to group B. The mean erythroid values for MCHC and MCH decreased significantly in group A when compared to group B. The mean haemoglobin concentration of group A was significantly lower when compared to that of group B. White blood cell count of group A decreased significantly (p<0.05) when compared to group B on day 28. The result of the differential leucocyte count showed significant decrease in lymphocytes in the heat stressed group when compared to the unstressed for the entire period. Total serum protein significantly increased in group A when compared to group B on day 14. Alanine amino transferase significantly increase in group A when compared to group B on day 28. Alkaline phosphatase decreased significantly in group A throughout the study period when compared to group B. Aspartate - amino transferase increased significantly in group A when compared to group B on day 28. Serum cholesterol significantly decreased in group A on day 28 when compared to group B.