Conditioned taste aversions (C.T.As) established in rats to 0.1% sodium saccharin by intra-peritoneal injections of DL-fenfluramine hydrochloride (6 mg per kg) or d-amphetamine sulphate (2.0 mg per ke) were found to be significantly attenuated, but not abolished altogether, by chronic pretreatment (over 9 days) with the specific drug. Prior treatment with fenfluramine attenuated the aversive effects of amphetamine, but the converse was found not to be the case. These results are considered to refute the "Unnatural need state AND "Novelty" hypotheses of the effects of prior drug experience on the establishment of C.T.As. An alternative explanation of such effects in terms of tolerance is considered, and the possible relevance of the results to studies of drug abuse in humans discussed.