Attenuation of the GH and insulin-like growth factor I (IGF-I) axis in aging may be responsible for changes in body composition and metabolism. This relationship has been confirmed by studies of recombinant human GH replacement in aging men and women, but the adverse effects encountered limit its clinical utility. The use of GHRH or its analogs may be an alternative mode for restoring the GH-IGF-I axis in aging individuals. Here we report the endocrine-metabolic changes in response to a GHRH analog in age-advanced men and women. A single blind, randomized, placebo-controlled trial of 5 months duration was conducted. Ten women and 9 men between the ages of 55-71 yr self-injected placebo (saline) s.c. nightly for 4 weeks followed by 16 weeks of [Nle27]GHRH-(1-29)-NH2 at a dose of 10 microg/kg. Subjects underwent 12-h nocturnal (2000-0800 h) frequent blood sampling (10-min intervals) and 24-h urine collection at baseline, after 4 weeks of placebo injections, and after 16 weeks of GHRH analog administration. GH responses to GHRH analog and spontaneous GH pulsatility were assessed. Subjects were also monitored 2, 4, 8, and 12 weeks after commencement of GHRH analog treatment. Blood pressure, body weight, and fasting insulin and glucose levels were recorded at each visit. Serum concentrations of IGF-I, IGF binding protein-1 (IGFBP-1), IGFBP-3, GH-binding protein (GHBP), lipids, and safety laboratory tests (complete blood count and chemistry profile) were measured in fasting samples (0800-0900 h). Body composition was determined by dual energy x-ray absorptiometry scan, and skin thickness was measured at four sites, including the right and left hand and volar forearm, by Harpenden skin calipers. Insulin sensitivity was assessed by a frequently sampled i.v. glucose tolerance test. Quality of life parameters, including sleep, were evaluated through self-administered questionnaires. Nightly GHRH analog administration at 2100 h induced, within 10 min, an acute release of GH, which lasted for 2 h. The GH-releasing effect of GHRH analog was sustained during the course of the study. Compared with placebo, GHRH analog induced a significant increase in 12-h integrated nocturnal GH levels in women (P < 0.01) and men (P < 0.05). This was accompanied, within 2 weeks, by increased serum levels of IGF-I (P < 0.05) and IGFBP-3 (P < 0.001), but not IGFBP-1, which remained elevated for 12 weeks, returning toward baseline by 16 weeks in both genders. Within 4 weeks, GHBP concentrations were significantly increased (P < 0.01) in women, but not in men. Although blood pressure and body weight were unaffected, GHRH analog treatment resulted in a significant increase in skin thickness (P < 0.05) in both genders and increased lean body mass in men only (P < 0.05), with no other changes in body composition or bone mineral density in either gender. There was a trend for a positive nitrogen balance in both genders, which became significant (P = 0.03) when the data were combined. Fasting insulin and glucose levels were unaltered, but a significant increase in insulin sensitivity occurred in men (P < 0.05), but not in women. Assessment of quality of life parameters revealed a significant improvement in general well-being (P < 0.05) and libido (P < 0.01) in men, but not in women, and sleep quality was unaffected in both genders. The only adverse side-effect was transient hyperlipidemia, which resolved by the end of the study. We conclude that nightly administration of GHRH analog for 4 months in age-advanced men and women activated the somatotropic axis. Although an increase in skin thickness was found in both genders, increases in lean body mass, insulin sensitivity, general well-being, and libido occurred in men but not in women. These observations suggest that GHRH analog administration induced anabolic effects favoring men more than women. Further studies are needed to define the gender differences observed in response to GHRH analog administration.