The regional distributions and frequencies of argyrophil endocrine cells in gastrointestinal (GI) tract of Balb/c-nu/ nu mouse were studied using Grimelius silver stain after abdominal subcutaneous implantation of COLO205. The experimental animals were divided into two groups, one is non-implanted group (Sham) and the other is COLO205-implanted group. Samples were collected from GI tract (fundus, pylorus, duodenum, jejunum, ileum, cecum, colon and rectum) at 21 days after implantation of COLO205 cells (1x10(6) cell/mouse). In this study, argyrophil cells were detected throughout the entire GI tract with various frequencies regardless of implantation. Most of these argyrophil cells in the mucosa of GI tract were generally spherical or spindle in shape (open type cell) while cells showing round in shape (close type cell) were found occasionally in gastric and/or intestinal gland regions. The regional distributions of argyrophil cells in COLO205 were similar to those of Sham. However, significant decreases of argyrophil cells were detected in COLO205 compared to those of Sham except for the jejunum and ileum. In the jejunum and ileum, argyrophil cells in COLO205 showed similar frequencies compared to those of Sham. In the pylorus, the most dramatically decreasement of argyrophil cells were detected in COLO205 compared to that of Sham. Implantation of COLO205 tumor cell line induced severe quantitative changes of argyrophil cell density, and the abnormality in density of GI endocrine cells may contribute to the development of gastrointestinal symptoms such as anorexia and indigestion, frequently encountered in patients with cancer.