Reptiles typically heat faster than they cool, and this thermal hysteresis is believed to derive from physiological mechanisms that modulate heat exchange with the environment through changes in thermal conductance. The vascular system, by means of autonomic regulation, is proposed to affect thermal conductance of reptiles by two mechanisms: (1) adrenergic (i.e., sympathetic) control of the peripheral vasculature through α-adrenergic receptors is suggested to be of paramount importance by increasing skin perfusion during heating and reducing perfusion during cooling; (2) cholinergic (i.e., parasympathetic) control of pulmonary blood flow through the vagus nerve supposedly serves to shunt blood away from pulmonary circulation to avoid heat loss over respiratory surfaces. We investigated the efficacy of heat exchange during warming and cooling in South American rattlesnakes (Crotalus durissus) before and after pharmacological α-adrenergic blockage through phentolamine injection, as well as with and without the ability to control pulmonary blood flow. Snakes were free to thermoregulate throughout the whole experiment using an intermittent heating source (on:off = 12:12 h) in a walk-in climatic chamber at a constant room temperature of 16°C (thermal gradient when heating source was turned on was 18°C). All snakes warmed faster than they cooled and behaviorally thermoregulated to maintain body temperature (T_b) at approximately 28–30°C. Neither sympathetic modulation of peripheral vascular resistance nor cardiac shunt control caused differences in warming and cooling rates. In a parallel experiment, coiling behavior was demonstrated to have a small but significant effect on snake thermal dynamics, albeit insufficient to explain the large thermal hysteresis observed. These results indicate that thermal hysteresis in rattlesnakes is not significantly affected by autonomic regulation of blood flow distribution. However, other physiological mechanisms must be important components of body temperature regulation in the South American rattlesnake.