The hypothesis is formulated that ageing is mediated through alteration in a pacemaker system located in the hypothalamus and dependent on serotonin-thyroid hormone interactions. Data in support of this hypothesis include: imbalance in central nervous system (CNS) neurotransmitters with ageing, with serotonin dominance; impairment of hypothalamic neurosecretion; increased heterogeneity of thyrotropin forms secreted from the aged pituitary; and drug and dietary manipulation of CNS serotonin with consequent effects on endocrine function and homocostasis.