Abstract: The effect of water deprivation and rehydration on some of blood constituents during different seasons in two local sheep breeds of Saudi Arabia, Awassi and Najdi, was investigated. Water deprivation induced significant (P<0.01) elevations in packed cell volume, plasma osmolality, total protein, sodium, creatinine and urea levels which were progressively increased with advance of water deprivation. Moreover, these responses were more pronounced during summer. Glucose levels tended to increase only following three days of water deprivation during summer. Both breeds responded to water deprivation in a similar manner during winter and spring seasons. On the other hand, Najdi breed had consistently higher responses during summer dehydration period particularly for plasma sodium and urea. Rehydration resulted in decline in the determined parameters, but more time was needed to regain normal levels during spring and summer. These results might suggest that there was not any differences in withstanding ability to water deprivation during winter and spring between the two breeds, however Awassi breed might have a better tolerance to dehydration during hot summer.