The sensitivity to heat and radiation of 22 rectal cancer tissues obtained at biopsy was studied using the in vitro succinate dehydrogenase inhibition test. The succinate dehydrogenase activity of tissue fragments was assayed after exposure at 43 degrees C (hyperthermia) for 20 hours, to radiation of 6 Gy, and to both heat (43 degrees C) and radiation (6 Gy). The sensitivity to each treatment was estimated by the percentage of succinate dehydrogenase activity of the treated cells compared with that of control cells. The mean plus or minus standard deviation of succinate dehydrogenase activity after exposure to radiation, heat, and both heat and radiation, was 84.7 +/- 12.6 percent, 52.9 +/- 20.7 percent, and 46.8 +/- 20.7 percent, respectively. The succinate dehydrogenase activities of heat-treated cells and both heat- and radiation-treated cells were significantly lower than that of the radiation-treated cells (P less than 0.01). The succinate dehydrogenase activities of heat plus radiation treated cells were the lowest in tissues from cancer lesions. Although the number was small, there was a correlation between this test and clinical outcome in seven of nine cases. Thus, preoperative therapy of hyperthermia plus radiotherapy is expected to be effective for treating patients with rectal cancer.