The sensitivity of human gastric cancer tissue to 5-fluorouracil (5-FU) and its analogues 1-(2-tetrahydrofuryl)-5-fluorouracil (FT), UFT and 1-hexylcarbamoyl-5-fluorouracil (HCFU) was determined, using the in vivo subrenal capsule (SRC) assay. The relative variation of tumor size (delta TS/TS0) was calculated as follows: delta TS/TS0 = (TS6-TS0/TS0) x 100%, where TS6 was the tumor size on day 6 and TS0 on day 0. The chemosensitivity was considered to be positive when delta TS/TS0 in the treated group decreased to below -10%. For their cytotoxic effects, 5-FU analogues are converted to 5-FU and positive correlations were noted between the tumor sizes of 5-FU and its analogues (5-FU vs. FT, r = 0.737; 5-FU vs. UFT, r = 0.653; 5-FU vs. HCFU, r = 0.709), in gastric tissues from 22 patients. The means +/- SD of tumor size were -8.5 +/- 11.5% for 5-FU, -8.3 +/- 16.0% for FT, -18.1 +/- 15.8% for UFT and -13.7 +/- 13.4% for HCFU. Decrease in tumor size was marked in case of exposure to UFT, compared with that to 5-FU (p less than 0.001), FT (p less than 0.001) and HCFU (p less than 0.05). 18% were sensitive to UFT and resistant to 3 other drugs. Thus, UFT proved to be the most effective among 5-FU and its analogues for decreasing the size of gastric cancer tissues.