Concentrations of bioavailable testosterone decline by 1.0-1.5% annually as men age. Reduced systemic testosterone availability is associated with decreased muscle mass, strength and aerobic capacity, decreased bone-mineral density and increased risk of hip fracture, waning sexual interest, inpaired spatial cognition and increased risk of visceral obesity, impaired glucose tolerance and coronary artery disease. Luteinizing hormone (LH) secretion often fails in healthy older individuals. In addition, aging is marked by an acceleration of LH pulse frequency, loss of high-amplitude LH pulses and disorderly release of LH and testosterone, as measured by the approximate entropy statistic. The mechanisms that underlie such complex adaptations are not known, but appear to involve multiple loci of regulatory failure.