We applied a new ultrasonic probe, the Ultrasonic micro-probe (UMP) -Toshiba Co.-, in experimental and clinical study to evaluate the wall structure and the lesions of the GI tract through the working channel of a conventional endoscope. This UMP is equipped with a 10, 15, or 20 MHz radial micro scanner in the tip and can be introduced through an endoscopic working channel with a caliber of at least 2.8 mm. The probe visualized the 5-layer structure within the normal gastric wall of canine and human cadavor in the water bath experiments. When the mural defects of a different depth were assessed by the probe to clarify the sonoanatomical correlation, the ultrasonic layers closely corresponded to the constitution of the wall in the anatomical strata. In the clinical study, the examination with this probe was performed in 40 patients under endoscopic visual control. The probe could detect small submucosal vessels in all patients, and furthermore, at 20 MHz one sometimes visualized 9 layers in the normal gastric wall under immersion in 200 to 300 ml of water, making the apparent delineation of lamina muscuralis mucosae ("mm") apart from the mucosal layer. Sonographic measurements of tela submucosae ("sm") and tunica muscularis propria ("pm") were possible under air inflation with the formal endoscopic procedure, demonstrating thick "sm" and thin "pm" for the body, and vice versa for the antrum. Small mucosal and submucosal lesions were well visualized in relation to the mural layer structure. The transmural extension of the early gastric carcinoma was well evaluated. This new probe was considered easier to use, more accurate it is in aiming, and better it is at image quality for the visualization of the normal GI wall and small lesions in it, compared with the instruments of conventional endoscopic ultrasonography. Technical improvements, including the stability in image quality, ultrasound penetration, and the flexibility of the working shaft, are anticipated.