The chemosensitivities of 62 human colon cancer tissues, 67 rectal cancer tissues and 31 tumor-adjacent normal mucosal tissues were determined using the in vitro succinate dehydrogenase inhibition (SDI) test. These tissues obtained at the time of surgery were exposed to carboquone (CQ), adriamycin (ADM), mitomycin C (MMC), aclacinomycin A (ACR), cisplatin (DDP) and 5-fluorouracil (5-FU). The chemosensitivity was considered as positive when succinate dehydrogenase (SD) activity of the drug-treated cells decreased to below 50% of that of control cells, on day 3 of exposure. Decrease in the SD activity was noted in the colon cancer tissues, compared to the rectal cancer tissues, exposed to six antitumor drugs and in particular, to CQ (p less than 0.05), DDP (p less than 0.01) and ACR (p less than 0.05, one-sided paired t test). Decrease in the SD activity was noted in the tumor tissues, compared to the tumor-adjacent normal tissues, exposed to CQ, MMC and ACR (p less than 0.01). The sensitive rates were higher in the colon cancer tissues than the rectal cancer tissues, against all six antitumor drugs. Our findings show that the rectal cancer tissues are resistant to antitumor drugs, compared to the colon cancer tissues in vitro. When selecting antitumor drugs to treat patients with a rectal cancer, the assessment for chemosensitivity of the related tissues is crucial.