Cholinergic denervation of the hippocampal formation has been extensively studied in rodents but not in primates. Therefore we studied the changes in acetylcholinesterase histochemical staining of the hippocampus occurring after bilateral transection of the fornices in the cynomolgus monkey. Animals were sacrificed 1.5, 6, 13, and 23 weeks after surgery. We found a 40-50% reduction in the density of acetylcholinesterase-positive fibers in the four analyzed regions (dentate gyrus, CA3, CA1, and subiculum) 1.5 week after surgery and a 60-80% reduction at longer time intervals. The characteristic diffuse AChE staining found in hippocampi from control animals disappeared after fornix lesion, except in the inner third of the molecular layer of the dentate gyrus. We did not find any evidence of spontaneous cholinergic reinnervation over the 6-month period. Thus, as in rats, fornix lesion produces dramatic changes in hippocampal AChE staining, presumably caused by a massive cholinergic denervation. However, in contrast to rodents, spontaneous reinnervation does not seem to occur in the months following the lesion in primates.