The overall objective is to delineate the nature and extent of control exerted by major thermoreceptor areas in effecting the changes in heart rate (HR), iliac artery blood flow (IBF), and superior mesenteric artery blood flow (SMBF) that are produced during external heating of unanesthetized chaired baboons. The first set of experiments will seek to determine the role of skin temperature (Ts) and core temperature (Tc) changes in effecting the changes in IBF, SMBF, and HR during heat stress. Independent control of Tc will be obtained by use of a heat exchanger incorporated into a chronic femoral arteriovenous shunt, whereby Tc can be kept at control level during Ts elevation (external heating) or Tc can be elevated independent of Ts changes at both cool and elevated Ts levels. By these combinations of Tc and Ts, the relative importance of Ts vs. Tc in effecting the observed changes in IBF, SMBF, and HR, the nature of the Ts-measured variable and Tc-measured variable relationships (linear or otherwise), and the influence of Ts changes on Tc-measured variable relationship (i.e., altered slope and/or set point?) can be determined. The second set of experiments will seek to determine the relative importance of the forebrain area (includes preoptic-anterior hypothalamus) vs. the non-forebrain area of the body core in effecting the IBF, SMBF, and HR changes during heat stress. Temperature of the forebrain area will be controlled by use of a heat exchanger placed into chronic exteriorized catheter loops which have been incorporated into the common carotid arteries. Temperature of the remainder of the body core will be controlled via a heat exchanger in a femoral arteriovenous shunt. By elevating forebrain and non-forebrain core temperatures in isolation from each other, the above objective will be met. These independent manipulations will also be done at cool and high Ts levels to further elucidate the influence of skin thermoreceptors.