With the aim of adequately distinguishing gastric submucosal tumors from other lesions such as gastric polyps, gastritis, and protuberances due to extragastric pressure, a new method was developed. A water soluble contrast medium is endoscopically injected into a submucosal lesion followed immediately by the application of roentgenography, thereby obtaining X-ray pictures of the lesion through frontal and lateral projections. Since the contrast substance is water soluble, the method can visualize submucosal microchanges. Highly diffusible, the substance usually has a fading time of about 40 min, the present method is convincingly free of danger. This paper provides the results of basic research performed using adult mongrel dogs together with some comments on the applicability of the method in clinical use.