1. The influence of (+ )-amphetamine, given 1 min after each training session, on the performance of 124 rats in a Lashley III maze was measured every 48 hr. 2. The first three injections of the drug significantly improved the learning ability of naive rats. 3. With prolonged treatment, (+)-amphetamine strongly impaired the maze performance of these rats. 4. The chronic administration of (+)-amphetamine to previously trained rats produced the same adverse effect. 5. Amylobarbitone sodium given to previously trained rats 30 min before the training sessions completely blocked the adverse effect of (+)-amphetamine. 6. (+ )-Amphetamine did not produce impairment of performance when given chronically 30 min before training sessions, to previously trained rats.
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