Hyperthermia has been shown to have a detrimental effect on experimental and human neoplasms. A water bath immersion system is described to evaluate the effects of systemic hyperthermia (SH) on the growth patterns of the Morris hepatoma 7777 in male Buffalo rats. SH and anesthesia were observed to have no long-term detrimental effects on weight trends or chow consumption. Inhibition of growth was demonstrated for this experimental tumor model at extreme SH (41.5° to 42.0°C), and it was statistically different (P<0.01) from the patterns of tumor growth observed in controls and tumor-burdened animals treated with moderate SH (39.5° to 40.0°C). Cessation of extreme SH resulted in acceleration of tumor growth so that no difference in tumor volume or animal survival was identified. SH resulted in retardation of tumor growth patterns, but its effects were not sustained once treatments were stopped.