The complex picture of age-associated brain cholinergic deficiency in humans and animals, and the possibilities of correcting it, are presented in this article. The changes that occur during ageing and senile dementias in cholinergic neurones and receptors and in the release and synthesis of acetylcholine are described and discussed. The drugs that have so far been administered to humans to correct cholinergic deficiency are listed and the effects of cholinesterase inhibitors, nerve growth factor and phosphatidylserine are discussed in some detail.