In this study, we developed a differential integrator unit containing a window discriminator (WPI-121) and three integrators (sample/hold, Gould) to characterize the difference in spikes involved in sympathetic vertebral nerve activity (VNA) and integrated VNA (Int. VNA) during stimulation of the dorsomedial (DM) and rostral ventrolateral (RVLM) medullar. The upper level (UL) in the window discriminator was set at a value, 80% of the VNA amplitude above the zero. The low level (LL) was set at a value 10% above the zero. The number of VNA spikes above the UL in the window discriminator was defined as Fa, while the number of VNA spikes within the UL and LL as Fw. In 26 cats anesthetized with urethane and alpha-chloralose, results showed that electrical stimulation and microinjections of glutamate (Glu, 0.25 M, 50 nl) or glycine (Gly, 1.0 M, 50 nl) at the same point in DM or RVLM increased both systemic arterial pressure (SAP) and Int. VNA. However, the increase of Int.VNA on DM stimulation was contributed by increases of both Fa and Fw, while on RVLM stimulation the increase of Int.VNA was mainly contributed by the increase of Fa, against a decrease of Fw. As compared to Glu, stimulation of DM by Gly produced more increase in Fw, while stimulation of RVLM by Gly produced more decrease in Fw. During RVLM stimulation the VNA was usually transferred to a huge synchronized oscillation. These findings suggest that although stimulations of the sympathetic-pressor neurons in DM and RVLM produce similar increases in SAP and VNA, the nature of these neurons is different.