Recent studies in children suggest that there are age-related differences in the insulin-like growth factor I (IGF-I) response to malnutrition. To extend this observation, immature 4-wk-old male rats were fasted for 3 days, fed ad libitum (control), or fed 60 or 40% of control calories (restricted) and compared with 8-wk-old young adults. Over the 3-wk study period, serum total IGF-I levels of the older rats were stable despite reduced insulin levels, whereas IGF-I increased 2.2-fold in the younger controls. With the 40% diet, younger and older rats changed body weight +1 and -1 body wt/day, respectively (P < 0.0001). The restricted younger animals reduced serum IGF-I IGF binding protein-3, acid-labile subunit, and growth hormone binding protein levels significantly more than the restricted older animals. Fasting decreased most of these parameters by 40%, serum insulin by approximately 80%, and body weight by 9%, regardless of age. We conclude that the suppression of the IGF-I system in response to chronic undernutrition, but not acute fasting, is greater in maturing than young adult rats.