Histaminase activity of several organs of a dog was determined by the volumetric method of Kapeller and Adler (1951), partly modified by the writer. In a normal dog, the activity of this eanzyme per gram of tissue was the highest in the kidney cortex, followed by the ileum and caecum, the duodenum showing lower activity (total layers used for the intestines), and the bladder, liver, stomach, lungs, skeletal muscles, and the skin all showed much lower values. Intravenous injection of a large dose of histamine or sinomenine caused decrease of histaminase activity only in the intestines and the liver, while daily subcutaneous injection of sinomenine for five days caused decrease of histaminase in the kidney cortex besides the above organs. Intravenous injection of peptone or corbicula extract, or anaphylactic shock caused marked decrease of histaminase activity in all these organs, but there was no such effect in ATP. Histamine, peptone and corbicula extract caused decrease of histaminase in the kidneys and intestines on perfusion of these organs. These facts indicate that the histaminase of these organs is capable of being released by certain substances and that these substances have a direct action of its own, besides that through the liberation of histamine that has the activity of liberating histaminase. Total adrenalectomy evidently decreased histaminase activity of the kidney cortex and the intestines.