Green tea has been incidentally found to produce a high signal on T1-weighted MR images. This study was therefore designed to investigate what substance in the green tea produced this T1-shortening effect and whether a green tea solution could be used as a positive enhancing agent in MR studies. As a result of the chemical analysis and measurement of T1 relaxation times in various solutions originating from green tea, it was found that the T1-shortening effect was produced not only by free manganese, but also by a high molecular weight substance (molecular weight 1.2 x 10(4) or more) united with manganese. In clinical MR studies, the gastrointestinal tract was markedly enhanced after oral administration of a green tea solution. The pancreatic head was clearly delineated with visualization of both the gastric wall and descending portion of the duodenum. However, visualization of the jejunum and ileum was not improved, suggesting that the green tea solution was diluted by intestinal fluid in the small bowel. Since only a small amount of manganese (1500 ppm) united with a high molecular weight compound such as pectin was sufficient to produce excellent enhancement of the gastrointestinal tract, it would be better to develop a purely synthetic chemical compound, for example, one consisting of a polysaccharide and manganese, rather than to use a green tea solution as a gastrointestinal enhancing agent in MR studies.