Circulating IGF-I is primarily regulated by growth hormone, but other factors such as nutritional status may also influence IGF-I secretion. The effects of age, gender, and estrogen replacement on responsiveness of serum IGF-I to GH administration have not been directly studied. The high-affinity circulating GH-binding protein (GHBP) has the same structure as the extracellular domain of the GH receptor and may reflect the sensitivity to GH in humans. To examine these issues, we employed an IGF-I generation test in which a single dose of GH (0.1mg/kg SQ) was administered to 31 healthy adults comprising five groups: young (20-29 years) males (YM), young females in the follicular phase of the menstrual cycle (YF), older (60-69 years) males (OM), older females not on estrogen replacement (OFN), and older females on oral estrogen replacement (OFE). Blood was sampled for IGF-I and GHBP over 72 hours following GH administration. OM had lower peak IGF-I levels (323 ± 38 vs. 497 ± 85 μg/l, p = 0.0015) and a lower IGF-I response to GH (ΔIGF-I: 187 ± 33 vs. 293 ± 57 μg/l, p = 0.0085) than YM. OFE had lower basal IGF-I (63 ± 11 vs. 133 ± 19 μg/l, p = 0.0046), peak IGF-I (174 ± 28 vs. 400 ± 40 μg/l, p = 0.0015), and ΔIGF-I (111 ± 21 vs. 268 ± 27 μg/l, p = 0.0085) than OFN. IGFBP-3 levels were unchanged 24 hours after GH administration. GHBP levels fell transiently in all subjects by an average of 22.9 ± 1.7%, with no differences among the groups. There was no correlation between IGF-I response and basal GHBP levels (r = 0.169, p>0.35). We conclude that (1) a single subcutaneous dose of GH is a convenient and direct method for assessing responsiveness to GH, (2) aging in males and oral estrogen therapy in postmenopausal women are accompanied by resistance to GH, (3) GH administration acutely lowers circulating GHBP, and (4) the level of circulating GHBP does not predict the IGF-I response to GH.