The medial septum is rich in cholinergic receptors and is a target for the development of cognitive enhancers. Intraseptal cholinomimetics have produced both promnesic and amnesic effects. Several variables (e.g. age, task difficulty) may influence treatment outcome. The present study examined the effects of intraseptal tacrine in a group of young cognitively impaired rats. These rats had been culled from a difficult radial maze task because they could not achieve criterion performance. Tacrine (0-12.5 microg/0.5 microl) enhanced radial maze performance in these animals. This effect contrasts with findings that intraseptal choliomimetics often have no effect or disrupt performance in young rats. Understanding the conditions in which cholinomimetics are promnesic is important for the further development of cognitive enhancers.