The effects of dehydration on vasopressin and oxytocin immunoreactive neurons in the hypothalamus was investigated by using a immunohistochemistry. Adult Mongolian gerbil[Meriones unguiculates] were deprived of drinking water. Dehydrated animals were sacrificed on the 7th, 14th and 21st day of water retriction. The results are as follows : 1. The body weights were decreased about 1.8% daily. On the 21st day of dehydration, they were shown up to 45% compare to the control. 2. In the hypothalamus of the control group, majority of vasopressin and oxytocin immunoreactive neurons were located in the supraoptic and paraventricular nuclei. 3. Changes due to dehydrated stimulation were mainly observed in vasopressin immunoreactive neurons. And these changes in supraoptic nuclei were more severe than those in paraventricular nucleus. Size of vasopressin immunoreactived cells and of areas were increased as to proceed the dehydration. The numbers of those were increased on the 7th day of dehydration, and then they were continously decreased. 4. Although oxytocin immunoreactive neurons were slightly changed in numbers during dehydration, they were not shown conspicuous changes compare to vasopressin immunoreactive neurons. Thus it is appeared that vasopressin secretory neurons in the hypothalamus of Mongolian gerbil are affected by osmotic stress induced dehydration while oxytocin neurons may be affected by other factors.