In dogs and humans, heat stress is associated with an increase in cardiac output that sustains blood flow to heat-dissipating organs. Because cardiac output and venous return are equal in the steady state, the circulation must also adjust in heat stress to allow the venous return to increase. To analyze these adjustments, we measured blood volumes, unstressed volumes, blood flow distribution, venous compliance, venous resistance, and the time constant of venous drainage of the splanchnic and extrasplanchnic vascular beds in dogs anesthetized with alpha-chloralose at normal and at high core temperatures. We repeated the measurements at high core temperatures with ganglionic blockade, alpha-adrenergic receptor blockade, or beta-adrenergic receptor blockade to determine the efferent neurohumoral pathway. When core temperature was increased from 37.8 +/- 0.2 to 41.9 +/- 0.1 degrees C, total splanchnic blood volume decreased 23% (4.6 +/- 1.4 ml/kg) and splanchnic unstressed volume decreased 38.5%. None of the other determinants of venous return changed. Ganglionic blockade shifted the total and unstressed splanchnic blood volume during heat stress back to normothermic values. However, beta- and alpha-blockade did not affect splanchnic volumes. We conclude that a decrease in splanchnic unstressed volume is an important factor for the increased venous return during heat stress. Although mediated through sympathetic ganglions, this decrease is not abolished by alpha- or beta-receptor blockade.