To evaluate possible therapeutic benefits of irradiation with far-infrared rays (FIR) on breast cancer, we examined combined effects of the chronic exposure to FIR at ambient temperature (26.5-27.5 degrees C) and the whole-body hyperthermia induced by FIR (WBH) (35-41 degrees C) on the growth of spontaneous mammary tumours of mice. A high mammary tumour strain of SHN virgin mice born on the normal rack or FIR rack were maintained on the respective racks until mammary tumour appearance. When the mammary tumour size reached approximately 7 mm, some mice in each group received no further treatment (Control and FIR groups, respectively) and the remaining mice received 3 hours of WBH each of 5 consecutive days (C + WBH and FIR + WBH groups, respectively). There was little difference between the control and FIR groups in the tumour growth over 10 days of examination. On the other hand, the tumour growth was inhibited significantly in both C + WBH and FIR + WBH groups and the degree of inhibition was similar. The data confirmed that the chronic exposure to FIR at ambient temperature has little effect on the growth of spontaneous mammary tumours in mice. WBH with FIR, however, strongly inhibited the tumour growth without deleterious side-effects, while chronic FIR irradiation itself again had little effect in this process. This WBH regimen may serve as a useful animal model for long-term studies of a noninvasive treatment of breast cancer.