SAWKA, MICHAEL N., RONALD G. KNOWLTON, and JERRY B. CRITZ. Thermal and circulatory responses to repeated bouts of prolonged running. Med. Sci. Sports. Vol. 11, No. 2, pp. 177-180, 1979. The purpose of this study was to determine the thermal and circulatory responses of seven runners (X Vo2max = 66 ml kg min) to two bouts of high intensity prolonged treadmill running (PTR) [each 80-min at 70% Vo2max] spaced by a 90-min rest. After the transition to exercise, oxygen uptake (Vo2) and percent change in plasma volume remained constant throughout each PTR. Rectal temperature (Tr) and heart rate (HR) increased while stroke volume (SV) and cardiac output (Q) decreased during the course of each PTR. The pattern of response for each variable was similar for both PTRs. However, Tr and HR were higher and SV and Q were lower during the second PTR when compared to the first at each respective time period.