Six buffalo heifers aging about 2.5 years and weighing 240-345kg at the beginning of the study were used to compare the physiological response (respiration rate, rectal temperature, serum bicarbonate and haemoglobin concentrations and haematocrit) in relation to acid-base balance under natural climatic conditions in winter and summer. All the Previous parameters were measured twice daily at 08.00 morning and 12.00 noon during each season where blood samples were collected at that times. Ambient temperature and relative humidity were recorded simultaneously. Haemoglobin concentration and haematocrit Percentages were lower in summer than in Winter (8.5 and 30.5 vs. 9.4 and 35.8, respectively), On the other hand a contradictory trend was observed concerning respiration rate, body temperature and serum bicarbonate, (38.3, 37.4 and 20.7 for winter vs. 42.6, 37.9 and 30.1 for summer, respectively). Bicarbonate concentration in the peripheral blood was 1.5 times higher in summer than in winter. The time of the day had an obvious effect on the physiological function since, body temperature and respiration rate were higher at 12.00 noon than the corresponding values of 08.00 morning in both seasons. The difference between the morning and noon values of respiration rate, rectal temperature and serum bicarbonate was greater in summer than in winter. An opposite trend was observed for haemoglobin concentration and haematocrit percentage