The ventrobasal complex (VB) of the thalamus was monitored in awake rats for the presence of norepinephrine (NE) overflow following pharmacological manipulations and physiological stimulation. Overflow was detected using chronoamperometry with electrochemically pretreated, Nafion-coated carbon fiber microelectrodes. In vivo evaluation of the electrode responses to systemic drug administration showed that alpha-methyl-p-tyrosine (alpha-MPT) and FLA-63 caused decreases in baseline current. Increases in baseline current in the VB were observed in animals treated with pargyline, yohimbine and yohimbine injected 2 h postpargyline. The results suggest that an electrochemical signal primarily due to NE overflow can be monitored in thalamic regions. Vigorous somatosensory stimulation induced small, long-lasting (approximately 30 min), reproducible electrochemical signals in the VB which were suppressed by alpha-MPT or FLA-63. These studies provide in vivo evidence which suggests that stressful somatosensory input to the VB initiates the release of NE.