Intracranial injection of 0.5 M hydroxylamine causes long lasting amnesia even when injected three weeks after training of passive avoidance task, alimentary conditioning or discrimination in water maze. Other functions of the brain and new learning are not affected. There is no spontaneous return of the erased memory. The longer is the interval between the learning and intracranial injection, the later the ammesia takes place. It is probably effective throughout the whole time of the existence of the memory trace, and it fails to cause amnesia only shortly before the natural extinction of the memory trace. The effect of the drug is probably not caused by the general illness of the animals, exhaustion or methemoglobinemia. Retraining of the same task as well as training of another task is possible. Our working hypothesis is that hydroxylamine affects memory through the effect on DNA.