Effects of Alismatis water extract (AE) on the blood pressure were investigated in the rabbit and the dog. AE, when administered into the vein of rabbit and dog, into the lateral ventricle of rbbit, produced fall of blood pressure. The depressor response of the rabbits to intravenous AE was abolished by treatment with atropine, but not with chlorisondamine weakened the depressor effect of AE. Intravenous AE in this rabbit produced secondary elevation of the blood pressure. AE potentiated the pressor response of the rabbit to norepinephrine and tyramine, but nor to angiotensin, acetylcholine, and DMPP.