Seventeen male hooded rats were trained on a CAR schedule in a shuttle box, using sound as the CS and shock as the US. After training, nine animals were injected intraperitoneally with 25 mg/kg mescaline hydrochloride, and eight animals with 25 mg/kg followed by 12.5 mg/kg. The results were recorded in terms of the number of shocks received, the number of crossings made and the reaction time from the onset of the CS. The results showed that mescaline exerted a biphasic effect, initially depressing the CAR, and then giving rise to a prolonged excitatory phase. When the experimental series was repeated two weeks later, it seemed that, although the behavioural pattern was basically the same some. tolerance of the drug had persisted. Further tests showed that this tolerance has disappeared eight weeks after the last injection. The smaller dose depressed the CAR less, and in fact the CAR was not depressed in less sensitive (Class II) animals, whereas the excitatory phase was increased and significantly so in Class II animals. Tests with trimethoxyphenylalanine indicated that this compound did not appear to affect CAR behaviour, even in doses of 100 mg/kg.