The variations in blood constituents of dairy cattle, bovine and buffalo calves as a result of exposure to varying environmental climatic conditions during the different seasons have been studied. Highly significant seasonal variations were noticed : 1.-Haemoglobin content was definitely increased during summer months, when the enviromental temperature increased in this part of Upper Egypt to 29°C and the air was comparatively dry (relative humidity 55.5%). Its content in buffalo-calves remained higher throughout the four seasons when compared with its value in other animals investigated, which may explain the increased adap tability of these animals to hot weathers. 2. A reduction in circulating erythrocytes was observed during summer, which may be probably a haemodilution effect brought about by the consumption of large amounts of water every day during exposure to high environmental temperatures. 3. The highest leucocytic count was observed during autumn and winter in all farms ivestigated, which by virtue of their phagocytic properties, they may assist in arresting the invasion of pathogenic micro-organisms during exposure to cold and windy weather. 4. Polymorphs, monocytes and basophils were found to be much higher during cold season. 5. The percentage of eosinophils was much increased during sum mer an autumn, indicating that the animals were not under heat stress during rise of environmental temperature.