The discharge characteristics of mechanoreceptors located in the left superior vena cava were determined in an in vitro preparation developed from the rat. Two classes of receptors with afferent fibers in the vagus were identified on the basis of their response to steps of pressure. Slowly adapting receptors (n = 18) discharged as long as pressure was above threshold and exhibited a biphasic decline in discharge frequency in response to a pressure step. There was a rapid, initial decline in discharge frequency, complete within 15 seconds, followed by a slow, gradual decline complete within 5 minutes. Rapidly adapting receptors (n = 15) discharged irregularly or ceased firing after 5-10 seconds of a step increase in pressure. Following a 15-minute increase in perfusion pressure from 0 to 5 mm Hg, the threshold pressure of the slowly adapting receptors was increased and maximal discharge frequency and slope were decreased, whereas, in the rapidly adapting receptors, the threshold pressure was increased but maximal discharge frequency and slope were unchanged. Additional perfusion of the vessel at 5 mm Hg for up to 60 minutes produced no further increase in the degree of resetting. The resetting was reversible with discharge returning to control levels after a 15- to 25-minute return to 0 mm Hg. To determine whether the resetting of discharge following the acute pressure increase reflected a change within the receptor or an alteration of the vessel wall due to sustained distension, we examined the passive mechanical properties of the superior vena cava. Pressure-radius relationships were determined with an ocular micrometer. A 15-minute increase in pressure had no significant effect upon the mechanical properties of the vessel. Rapid resetting in these low pressure mechanoreceptors occurs with no discernible alteration in the vessel wall.