Reproductive hormone release following kisspeptin-54 in young and older men. Mean ± standard deviation of serum hormonal response to kisspeptin-54 0.1, 0.3, and 1 nmol/kg/h in young (n = 10) (a–c) and older men (n = 5/group) (d–f). Serum LH (IU/L) is shown in blue, serum FSH (IU/L) in red, and serum testosterone (nmol/L) in purple. FSH, follicle-stimulating hormone; KP54, kisspeptin-54; LH, luteinising hormone; Testo, testosterone.