To determine if classical conditioning produces general or specific modification of responses to acoustic conditioned stimuli (CS), frequency receptive fields (RF) of neurons in guinea pig auditory cortex were determined before and up to 24 h after fear conditioning. Highly specific RF plasticity characterized by maximal increased responses to the CS frequency and decreased responses to the pretraining best frequency (BF) and other frequencies was observed in 70% of conditioning cases. These opposing changes were often sufficient to produce a shift in tuning such that the frequency of the CS became the new BF. CS frequency specific plasticity was maintained as long as 24 h. Sensitization training produced general increased responses across the RF without CS specificity. The findings indicate that associative processes produce systematic modification of the auditory system's processing of frequency information and exemplify the advantages of combining receptive field analysis with behavioral training in the study of the neural bases of learning and memory.