In aseries of four experiments, the effects of the reactivation of an old memory in different states of vigilance were investigated. Rats were trained to run in a six-unit spatial discrimination maze for food reward with low-Ievel electrical stimulation delivered to the ear when they entered blind alleys. Twenty-five days after training, electrotactile stimulation was given to animals as a reactivating treatment just before a retention test. When the cuing treatment was given during wakefulness, retention performance was improved. When it was given in slow-wave sleep, it had no effect. When it was given in paradoxicalsleep, performance was poorer. The use of control groups permitted an assesament of the specificity of the cuing effect in each case: Pretest presentation ofthe same stimulus did not modify performance when it had no cue value. Together, these results show that the effect of the reactivation of an old memory depends upon the vigilance state in which reactivation takes place. They suggest that reorganization of an old memory could be accomplished during paradoxical sleep.