Effects of aging and sex on total plasma IGF-I levels were studied in 207 normal adults (103 males and 104 females), aged 21 to 80 years. Plasma IGF-I concentrations were measured by specific radioimmunoassay after extraction with acid-ethanol. Plasma IGF-I levels ranged from 50.8 to 480.6 μg/l with a mean (±sd) value of 200.7±81.9 μg/l in these normal adults. The mean plasma IGF-I levels were higher in females than in males in the second decade (332.6±63.9 vs 252.4±45.2 μg/l, p<0.01), whereas there was no sex difference in plasma IGF-I levels in elderly subjects. In both sexes, plasma IGF-I levels declined with age. In females, the slope of the regression line was much steeper in the young group (aged 21 to 40 years) than in the middle-aged and elderly group (aged 41 to 80 years) (y=−11.3x+641.7, r=−0.538, p<0.005 and y=−1.97x+289.3, r=−0.308, p<0.01). In males, plasma IGF-I linearly declined with age and the slope of the regression line (y=−2.36x+322.2, r=−0.480, p<0.005) was not considerably different from that of the middle-aged and elderly group in females.