In my previous study it was found that the water metabolism of pyridoxine deficient rats was significantly impaired. Since 2-methyl-4-amino-5-hydroxymethylpyrimidine (OMP) has been shown to be antagonistic to pyridoxine. The water metabolism of OMP-treated rats was studied. These rats exhibited a delayed response to a water load. However, the ability of liver and kidney vasopressin was not impaired at all, and the content of ADH in the pituitary gland also unchanged. These results were in accord with those observed in pyridoxine deficient rats. It was inferred that ADH does not play a significant role in the derangement of water metabolism. On the other hand, OMP treatment casued a decrease in adrenal ascorbic acid content and adrenal weight ; these findings were different from those in pyridoxine deficient rats. It was suggested that adrenocortical insufficiency resulted from OMP treatment might be one of the causes of the derangement of water metabolism.