The effects of variations in the amount of training on behavioral plasticity were examined in three experiments that used appetitive Pavlovian conditioning procedures with rats. In experiments 1 and 2, an auditory conditioned stimulus (CS) substituted for a food unconditioned stimulus (US) in the acquisition of new learning about the food US after small numbers of CS-US pairings, but not after larger numbers of pairings. After limited exposure to the relation between the auditory CS and food, pairings of the CS with the toxin LiCl, in the absence of food, were sufficient to establish an aversion to the food US that was previously paired with that CS. This CS-mediated learning did not occur after more extensive exposure to the CS-food relation. In contrast, in experiment 3. mediated performance of previously-established conditioned responding was unaffected by the number of CS-US pairings used to establish that performance. Conditioned responding to the auditory CS remained sensitive to post-training devaluation of the food US regardless of the amount of initial CS-US training. Implications of these results for the investigations of cortical and other brain mechanisms of behavioral plasticity are discussed.