The pattern of neuronal activation in the rat nucleus of the solitary tract (NTS) in response to a standard gustatory stimulus was examined using c-Fos-like immunoreactivity (c-FLI) before and after conditioned taste aversion (CTA) formation. While unconditioned oral infusions of sucrose solution did not induce c-FLI in the NTS, after three pairings of sucrose with lithium chloride injections, sucrose induced c-FLI in the medial intermediate NTS 1 h after oral infusion. Extinction of the CTA by repeated infusions of sucrose alone reversed the induction of c-FLI.