Context: Circulating testosterone (T) and GH/IGF-I are diminished in healthy aging men. Short-term administration of high doses of T augments GH secretion in older men. However, effects of long-term, low-dose T supplementation on GH secretion are unknown. Objective: Evaluate effects of long-term, low-dose T administration on nocturnal GH secretory dynamics and AM concentrations of IGF-I and IGFBP-3 in healthy older men (65-88 yr, n=34) with low-normal testosterone and IGF-I. Design: In a double-masked, placebo-controlled, randomized study, we assessed effects of low-dose T supplementation (100 mg IM every 2 weeks) for 26 weeks on nocturnal GH secretory dynamics (8 PM-8 AM, q20 min sampling, analyzed by multiparameter deconvolution and Approximate Entropy (ApEn) algorithms). Results: T administration increased serum total T by 33 % (P = 0.004) and E2 by 31% (p=0.009), and decreased SHBG by 17% (p = 0.002), versus placebo. T supplementation increased nocturnal integrated GH concentrations by 60 % (P = 0.02), and pulsatile GH secretion by 79%, (P = 0.05), primarily due to a two-fold increase in GH secretory burst mass (P = 0.02), and a 1.9 fold increase in basal GH secretion rate (P = 0.05), versus placebo. There were no significant changes in GH burst frequency or orderliness of GH release (ApEn). IGF-I levels increased by 22 % (P = 0.02) with no significant change in IGFBP-3 levels, after T versus placebo. Conclusions: Low-dose T supplementation for 26 weeks increases spontaneous nocturnal GH secretion and morning serum IGF-I concentrations in healthy older men.