The effect of selank (Thr-Lys-Pro-Arg-Pro-Gly-Pro), which is a synthetic derivative of the endogenous tetrapeptide tuftsin (stable with respect to tissue peptidases), on the learning and memory processes and metabolism of serotonin (5-HT) have been experimentally studied on Wistar rats. The animals were trained with food reward in 30 trials per day. Selank (300 microg/kg) or saline were injected after the 10th trial. Elaboration of conditioned reflex with food reward was continued 30 min later. Retention was tested 24 h, 7 and 30 days after treatment. A single injection of selank activated the metabolism of 5-HT in the hypothalamus and caudal brain stem for 30 min to 2 h. It was established that selank induces an increase in memory trace stability during 30 days. These findings provide direct evidence that selank, when injected during consolidation phase, can enhance memory storage processes. Nootrope activity of the selank is probably caused by its obvious effect on the level of serotonin and its metabolite in the brain.