0.25M sucrose homogenate of dog's liver was fractionated by differential centrifugation as described by Schneider. About half of histamine of the whole homogenate was found in the mitochondrial fraction. Smears were made from each fractions, fixed in 1 per cent basic lead acetate in 50 per cent alcohol and stained with 0.1 per cent aqueous solution of toluidine blue. Microscopic observation showed that the mitochondrial fraction contains a large amount of very small round metachromatic particles, which are indistinguishable from extruded granules outside of the mast cells similarly stained in the stamp preparation from the liver slice. These observations confirm the finding of Mota and his co-workers and this fact in all probability indicates that the higher histamine concentration of the mitochondrial fraction of dog's liver might be due to contamination with mast cell granules. Intravenous administration of 3 mg/kg sinomenine or 150 to 500 mg/kg peptone into dogs resulted in a significant release of histamine from the mitochondrial fraction with some variable loss from other subcellular fraction.