The effects of prior irradiation (8 or 10 Gy of X rays) on the response of mouse jejunum to a test treatment of (a) hyperthermia (42.0 degrees C for 60 min), (b) X rays (8 or 10 Gy) or (c) combined hyperthermia and radiation were investigated at various times up to 7 months after the primary treatment. Tissue injury was quantified by crypt survival 4 days after the test treatment. Although tissue susceptibility to direct thermal injury was increased 10-15 days after irradiation, neither radiation sensitivity nor thermoradiosensitisation were significantly affected at any of the time intervals used. The results support the concept that there are different pathways for the in vivo expression of gross thermal damage and thermoradiosensitisation.