The circulatory reactions during radiothermia (i.e., hyperthermia produced by passing short radio waves through the body) were investigated in anesthetized dogs by registering the pressure pulses from an innominate artery by a mirror manometer. Several types of experiments were carried out: (1) heating by radio waves continued until death; (2) radiothermy interrupted for varying periods and at different temperature levels; (3) hyperthermia maintained between 39.8 to 41[degree] for 1-2 hrs., and the recovery studied. Qualitative and quantitative studies of the circulatory reactions reveal certain differences in hyperthermias produced by high external temperatures and by radiothermy. In hyperpyrexia due to the former, blood pressures tend to decline above 41[degree] C. The pressure pulses become smaller, rise more gradually to a peak and are devoid of all secondary vibrations. In short, they display features which suggest that systolic discharge is reduced and the force of ventricular contraction decreased. During radiothermia, on the other hand, systolic and diastolic pressures increase tremendously after heart temperature exceeds 41[degree] C. The pressure pulse increases greatly in amplitude, rises sharply to a peak and contains many after-vibrations. All of these features suggest that the force and vigor of ventricular contraction is improved until the crisis is reached. Many features of the curves suggest, however, that the systolic discharge delivered by each beat may not be increased and perhaps is decreased. Vascular changes doubtless contribute to the blood pressure changes, but whether these are in the nature of an increase in peripheral resistance or a decrease in elasticity of vessel walls requires further study. Though changes in heart rate, pulse pressure and pulse form have been observed without changes of temperature within the heart cavities, this does not prove that the passage of radio waves exerts an effect independent of local temperature changes in the cardiac or vascular structures. When radiothermy is continued long enough, death occurs as in other forms of hyperthermia, i.e., through primary failure of the respiratory center which is followed by circulatory failure due to asphyxia. Animals kept at 39.8-41[degree] for 2 hrs. manifest a circulatory reaction as the temperature declines and returns to normal. The blood pressures fall and pressure pulses clearly indicate that systolic discharge is decreased and ventricular expulsion less forceful. These reactions combined with vascular relaxation may cause hypodynamic states which, though not to be classed as circulatory failure, are nevertheless severe reactions. Hence the suggestion is made that great care and caution is advisable in men and animals exposed to prolonged radiothermy.