Fear conditioning modifies the processing of frequency information; receptive fields (RF) in the auditory cortex and the medial geniculate body (MGB) are altered to favor processing the frequency of the conditioned stimulus (CS) over the pretraining best frequency (BF) and other frequencies. This experiment was designed to determine whether brief conditioning in the waking state produces RF plasticity that is expressed under general anesthesia. Guinea pigs bearing electrodes in the MGB received 20 trials of tone-shock pairing in a single training session. RFs were determined with animals under ketamine anesthesia before conditioning and 1-3 hr and 24 hr after conditioning. Frequency-specific RF plasticity was evident for both postconditioning periods: The BF shifted toward or to the CS frequency, responses to the BF decreased, and responses to the CS increased. Broadly tuned cells developed greater RF plasticity than narrowly tuned neurons. The results demonstrate that the specific neuronal results of brief learning experiences can be expressed in the anesthetized brain.