The overall objective is to delineate the afferent and efferent mechanisms of control of heart rate (HR), iliac artery blood flow (IBF) and superior mesenteric artery blood flow (SMBF) during external heating of chronically instrumented baboons. These 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. From these combinations of Tc and Ts, one can determine the relative importance of Ts vs. Tc in effecting the observed changes in IBF, SMBF and HR, and the influence of Ts changes on the effector response-Tc relationship (i.e., altered set-point and/or slope?). In addition, these experiments will seek to determine the relative importance of direct sympathetc innervation of the intestinal vascular bed vs. the renin-angiotensin system in effecting the intestinal vasoconstriction during heat stress. These studies will be done on baboons exposed to heat stress following pharmacologic blockade of alpha-adrenergic receptors, beta-adrenergic receptors and angiotensin II receptors.