The influence of the activities of daily living on human growth hormone (hGH) release and plasma insulin-like growth factor (IGF-I) levels is not known. Individuals with spinal cord injury (SCI) and paralysis generally have reduced levels of activity compared with ambulatory subjects. We studied sixteen subjects with SCI and sixteen nonSCI subjects matched for age, gender and body mass index (BMI) as controls. After an intravenous infusion of arginine hydrochloride (30 g/subject over 30 minutes), mean plasma hGH values at 30 and 60 minutes were significantly lower in the group with SCI compared with the control group (3.4 +/- 0.7 versus 10.7 +/- 2.5 ng/ml, p < 0.01; and 5.2 +/- 1.5 versus 12.5 +/- 2.7 ng/ml, p < 0.05). Also, peak and sum hGH responses were significantly lower in the group with SCI than in the control group (5.8 +/- 1.5 versus 14.1 +/- 2.8 ng/ml, p < 0.01; and 15.2 +/- 3.1 versus 34.8 +/- 7.2 ng/ml, p < 0.02). Controlling for age and BMI, the results remained significant. However, the mean plasma IGF-I level was significantly lower in SCI subjects younger than 45 years old than in the similar subgroup of age-restricted controls (202 +/- 19 versus 324 +/- 27 ng/ml, p < 0.05), whereas, a comparison of subgroups of subjects 45 years or older did not reveal a significant difference. These findings support the hypothesis that decreased daily physical activity results in depression of the hGH/IGF-I axis in younger individuals with SCI and may be considered to be a state of premature aging.(ABSTRACT TRUNCATED AT 250 WORDS)