Exercise training leads to tissue anabolism by acting through the growth hormone (GH)-insulin-like growth factor-I (IGF-I) axis, but the role of tissue IGF-I receptors in this process is not known. Erythrocyte IGF-I receptor binding is known to be affected by circulating levels of IGF-I. We hypothesized that training would lead to alterations in erythrocyte IGF-I binding. Thirty-eight adolescent males (mean age 16 ± 0.7 yr) were randomized to a control (n = 18) or endurance training intervention lasting 5 weeks. Erythrocyte IGF-I binding was measured by standard techniques. Quantity of receptor binding sites (R), binding affinity constant (K(aff)), and nonspecific binding (NS) were calculated. Training led to increases (p < 0.05) in 1) K(aff) x R - an index of overall binding capacity (control, 7.3 ± 8.0%; trained, 60 ± 21%); and 2) NS (control, 1.5 ± 6.6%; trained 23.2 ± 7.5%). K(aff), R, and R/erythrocyte increased in trained subjects, but not significantly so. These results suggest a training-associated upregulation of IGF-I binding. Finally, the unexpected increase in NS may reflect IGF binding protein activity, rather than changes in the erythrocyte IGF-I receptors themselves.