Cellular effects of a combined treatment with X-rays (15 Gy) and local microwave hyperthermia (43 degrees C, 60 min) on the tumor tissue of the rhabdomyosarcoma R1H of the rat were studied with light and electron microscopy. The results were compared with the cell reactions obtained after X-rays (15 Gy) alone. The histological results and the morphometric data show that the time course of cell damage changes individually with the treatment applied. After radiation alone a cell oedema occurs within some days. After a combined treatment, however, a marked cell oedema and cell lysis is already observed after 24 hours. An intensive shrinking of the cell nuclei is found at day 6th, attended with pyknosis, karyolysis and cell fragments in the extended extracellular area. In comparison with radiation alone the processes of repair and repopulation in the R1H tissue are remarkably delayed after a combined treatment.