Temperature was measured in the left ventricle, aorta, liver, brain, lung, bone marrow, kidney, and spontaneous solid tumors in dogs undergoing whole body hyperthermia in a radiant heat device. Rectal temperature was found to be a satisfactory indicator of systemic arterial temperature during plateau temperature conditions but rectal temperature underestimated arterial temperature during heating and overestimated it during cooling. Lung temperature, based on small airway temperature, was the same as rectal temperature during plateau temperature conditions. Liver and brain temperatures were slightly higher (0.1-0.2 degree C) than rectal temperature during the plateau phase. During plateau temperature conditions, kidney temperature measurements were higher than rectal temperature when one site/kidney was measured but were lower than rectal temperature when two sites/kidney were measured suggesting invasive thermometry may have affected measured temperature values. Tibial marrow temperature was greater than rectal temperature during heating but fell below rectal temperature during plateau temperature conditions by as much as 1.3 degree C. Femoral marrow temperature was below rectal during heating but gradually exceeded it during steady state conditions, by 0.1-0.4 degree C. Temperature in solid tumors was variable, sometimes exceeding (0.6 degree C) and sometimes being less (1.8 degree C) than rectal temperature.