Abstract-The rostral ventrolateral medulla (RVLM) is considered a major center for the regulation of sympathetic and cardiovascular activities. Several clinical studies have indicated a possible causal relationship between neurovascular contact of the left RVLM and essential hypertension, and some investigators have suggested that the left RVLM is more sensitive to pulsatile compression than the right RVLM. Previously, we reported that pulsatile compression of the RVLM elevates arterial pressure by enhancing sympathetic outflow in rats; however, we have not investigated the laterality of the responses to the compression. In addition, it remains to be elucidated whether RVLM neurons are activated by compression and, if so, how they are activated. Therefore, we performed compression experiments in rats to investigate these issues. Pulsatile compression was performed on the unilateral RVLM with a pulsating probe in anesthetized and artificially ventilated rats. Pulsatile compression of the unilateral RVLM increased arterial pressure, heart rate, and sympathetic nerve activity. The pressor response to compression was inhibited significantly after local microinjection of glutamate receptor antagonists. Pulsatile compression of the RVLM increased Fos immunoreactivitiy, a marker of neuronal activation, within the nuclei of postsynaptic RVLM neurons. All results were observed symmetrically. The data indicate that the responses to pulsatile compression of the unilateral RVLM are similar on both sides. They also suggest that pulsatile compression of the RVLM increases sympathetic and cardiovascular activities by activating postsynaptic RVLM neurons through the stimulation of the local glutamate receptors in rats. (Hypertension. 1999;33:1207-1213.)Key Words: neurons Ⅲ receptors, glutamate Ⅲ ventrolateral medulla Ⅲ sympathetic nervous system Ⅲ arterial pressure T he rostral ventrolateral medulla (RVLM) contains neurons that are the major tonic source of supraspinal sympathoexcitatory outflow 1,2 ; thus, this area is considered an important center that regulates sympathetic and cardiovascular activities.It has been reported that the posterior inferior cerebellar artery, anterior inferior cerebellar artery, or vertebral artery occasionally compresses the medulla oblongata in humans. Several clinical and necropsy studies have indicated a possible association between essential hypertension and neurovascular contact of the ventrolateral medulla at the root-entry zone of the glossopharyngeal and vagus nerves (ie, at the RVLM). [3][4][5] We reported previously that the incidence of observed neurovascular contact of the RVLM in an essential hypertension group was significantly higher than that in a secondary hypertension group or a normotensive group by use of MRI with a high-resolution matrix. 6,7 Thus, we hypothesized that neurovascular contact of the RVLM might be, at least in part, causally related to essential hypertension.It has been shown that microvascular decompression of the RVLM on the left but not the right side improve...