Many in vitro effects of volatile anesthetics are known, but the mechanisms of action are still under debate. Because suppression of sensory perception is one of the major goals of general anesthesia, we studied the effects of isoflurane on the processing of somatosensory information in anesthetized rats. Local iontophoretic administration of the gamma-aminobutyric acid-A (GABA(A)) receptor antagonist bicuculline in the thalamic ventral posteromedial nucleus reversed suppressive effects of isoflurane on thalamocortical relay neurons (TCNs). The action potential discharges of TCNs (n = 23) in response to defined mechanical stimulation of receptive fields seen with small concentrations of isoflurane (0.79% +/- 0.01%, mean +/- SEM) were suppressed under large concentrations (1.44% +/- 0.04%). In addition, the tonic response pattern was lost, which initially encoded the information about the stimulus features. In 70% of TCNs, bicuculline administration reestablished the initially present tonic response pattern under large isoflurane concentrations. These results indicate that isoflurane suppresses somatosensory information transfer at the thalamic level in vivo, apparently by enhancing thalamic GABA(A) receptor-mediated inhibition.