Aim of the study. The aim of the study was to investigate in CAPD patients whether circulating insulin-like growth factor-I (IGF-I) bioactivity may offer a more sensitive index to acute nutritional interventions than total IGF-I. Methods. An open-label, randomized, crossover study of 2 days-with a 1-week interval-was performed in 12 CAPD patients in the fed state to compare a mixture of amino acids (Nutrineal 1.1%) plus glucose (AA plus G) (Physioneal 1.36% to 3.86%) dialysate versus G only as control dialysate. Fed-state conditions were created by identical liquid hourly meals. IGF-I bioactivity was measured by the kinase receptor activation assay (IGF-I KIRA); total IGF-I was measured by immunoassay. Results. In the fed state, both after AA plus G as well as after G dialysis IGF-I bioactivity increased compared to baseline, while no changes in circulating total IGF-I levels were observed in both treatment arms. However, the increase in IGF-I bioactivity was only significant after AA plus G dialysis (P = 0.02). Conclusions. Our results provide evidence that in CAPD patients changes in circulating IGF-I bioactivity are associated with nutrient intake and that IGF-I bioactivity rather than total IGF-I is involved in acute responses to nutritional interventions in CAPD patients.