Abstract
Cats were trained to respond on a multiple discriminative schedule, with milk as reinforcement. Two subjects did not immediately consume the reinforcer when they were injected with 6 mg of methylphenidate before the experiment. This observation could be repeated in one of the subjects under various conditions of reinforcement and various doses of the drug. Control experiments showed that under normal conditions the same cats never ignored the reinforcer. The modification induced by the drug in the relationship between behavior and the reinforcement is discussed in its bearing on the notion of reinforcer.