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This study was conducted to investigate the effects of increasing dietary protein intake on growth performance and serum insulin-like growth factor-I (IGF-I) concentration in growing-finishing gilts. Thirty-nine crossbred gilts with an initial body weight of 74.3 lb were used in a 28-day growth study. The gilts were randomly allocated to one of five dietary treatments. The diets were standard corn soybean meal diets, which were formulated to contain 10, 14, 18, 22, or 26% crude protein by changing the ratio of corn to soybean meal in the diet. Pig and feeder weights were recorded weekly for the determination of average daily gain (ADG), average daily feed intake (ADFI) and feed efficiency (ADG/ ADFI). Weekly blood samples were collected to evaluate dietary effects on plasma urea and IGF-I concentrations. There was no difference (P > 0.10) in ADFI among the treatments throughout the 28-day experimental period. Dietary protein concentration had significant linear and quadratic effects on ADG and ADG/ADFI (P < 0.01). Gilts fed the diet containing 22% CP had the greatest accretion rate of fat-free lean (0.82 lb/d); however, gilts fed the 18 and 26% CP diets had numerically similar fat-free lean accretion rates. Increased dietary protein concentration resulted in increased cold carcass weight (linear, P < 0.01; quadratic, P < 0.01) with no differences in carcass dressing percentage. Protein concentration had a significant quadratic effect (P < 0.01) on plasma urea and serum IGF-I concentration during weeks 1 thru 4 of the experiment. In summary, dietary protein concentration had significant linear and quadratic effects on final body weight, ADG, feed efficiency, fat-free lean gain, cold carcass weight, plasma urea and serum IGF-I concentration. Thus, the interesting finding in this experiment was that the decrease in fat-free lean gain in gilts fed the 14% CP diet was not associated with a decrease in serum IGF-I concentration. This finding suggests that something is inhibiting the actions of IGF-I protein by causing a decrease in protein accretion rate in these gilts. Thus, the future focus of this research is to determine the effects of dietary crude protein and crystalline amino acids on serum IGF-I concentration.