In four conditioned taste aversion experiments with rats as subjects, the effects of extinguished or pre-exposed flavors on retardation and summation tests was compared. Experiment 1 showed that when steps were taken to ensure similar exposure to the target flavor in all conditions, acquisition after pre-exposure and reacquisition after extinction proceeded at a similar rate and was slower than acquisition in a new stimulus control condition. In Experiment 2, reacquisition occurring 2 days after extinction was again slower than acquisition to a new stimulus, but this retardation disappeared when extinction and reacquisition were separated by a 21 days interval. Experiment 3 showed that both an extinguished and a pre-exposed flavor produced a similar summation effect, attenuating the aversion to a previously conditioned flavor. Finally, Experiment 4 showed that this attenuation was also produced by a new flavor. These results suggest, first, that retarded acquisition after extinction of a conditioned taste aversion might be the result of latent inhibition produced by extended experience with the flavor during extinction and, second, that attenuation of aversion to a test excitor on a summation test might not reflect any specific learning process but be simply due to stimulus generalization decrement. (Contains 4 tables and 4 figures.)