In order to clarify the nature of the secretory process of water in dog's submaxillary glands, the following experiments were carried out:(1) measurement of the secretory pressure, (2) measurement of osmolarity of saliva secreted against a high intraluminal pressure, and (3) observation of the effect on the intraluminal pressure of retrograde injection of a hypertonic sucrose solution or an isotonic NaCl solution. It was found that when the secreted saliva was lead to a vertical polyethylene tube the secretion continued until the salivary column reached a height of more than 350 cm with continuous stimulation of the chorda tympani. The osmotic pressure of saliva was examined when it was secreted by stimulation of the chorda tympani into a closed luminal system with a pressure transducer. Under the influence of high luminal pressure (250 mmHg), the osmotic pressure was found to be 350 mOsm, the value being significantly hypertonic as compared with serum. By using the equation ΔP=δRTΔC, which is applicable to the gland system under zero flow conditions, the value of a of the system for salt (probably NaCl) was calculated to be about 0.23. From these findings, it is concluded that the osmosis across some parts of the gland, probably the acinus and the intercalated duct, is very important in water secretion of dog's submaxillary glands, as proposed in the osmotic flow model of Lundberg, Petersen, and Slegers and van't Hof-Grootenboer.