Monkeys with bilateral transection of the fornix were severely but selectively impaired on learning and retention of visuospatial conditional discriminations, visual conditional discriminations and non-conditional spatial-response tasks. Bilateral transplantation of cholinergic-rich fetal basal forebrain tissue into the hippocampus abolished significant learning impairments on all those tasks impaired by fornix lesions when tested three to nine months after transplantation whereas bilateral transplants of non-cholinergic fetal hippocampal tissue into hippocampus showed no such beneficial effect. Acetylcholinesterase staining was severely depleted throughout the dentate gyrus and hippocampus in fornix-transected monkeys compared with animals with control corpus callosum ablations. Staining was largely restored to normal in the host hippocampus and dentate gyrus in monkeys with cholinergic transplants, whereas acetylcholinesterase staining was abnormal in those with non-cholinergic grafts. These experiments suggest that where a "higher order" cognitive function, in this case the acquisition of specific types of information into long-term memory, is disturbed by a neuropharmacologically simple lesion, cognitive function can be restored by transplantation of neurons containing appropriate neurotransmitters.