Hippocampal, cortical and sham lesioned rats were trained to discriminate between two odors, in non-contingent, partially contingent, and perfectly contingent schedules. In the non-contingent situation both odors signaled shock with probability of 0.5. In the partially contingent situation one odor signaled shock with probability of 0.5 and the other "no shock" with probability of 1. In the perfectly contingent situation one odor signaled shock and the other "no shock" with probabilities of 1. In the non-contingent schedule none of the three surgical groups evidenced learning, in the partially and perfectly contingent cases the hippocampally lesioned animals performed so as to indicate greater learning of the aversion than did the cortically lesioned or the sham operated animals. It is suggested that the behavior of animals with hippocampal lesions is more sensitive to contingencies than that of normal animals.