The release of extracellular acetylcholine (ACh) in the insular gustatory cortex of conscious rats during taste stimulation was measured using the microdialysis technique. The mean basal release of ACh before stimulation was 273 +/- 21 fmol/10 microliters (mean +/- S.E.M., n = 25). Intraorally applied taste stimuli or distilled water significantly increased the release of ACh. Among them, infusion of 0.001 M quinine HCl produced a marked increase in the release of ACh up to 355% of baseline levels. Infusion of 0.01 M saccharin to the subjects that had acquired an aversion to this taste also caused a prominent increase in ACh up to 343% of basal levels. In contrast, saccharin infusion to the naive subjects moderately increased ACh up to 243% of baseline. Water infusion resulted in the smallest increase in ACh up to 175% of baseline. Although intraoral infusions of quinine or distilled water caused a significant increase in ACh in the parietal cortex, the magnitude of increased ACh was smaller than that in the gustatory cortex. These results suggest that ACh release in the insular gustatory cortex is related to behavioral expression to aversive taste stimuli.