The composition of enteral feeding formulas may have different effects on total body and gut growth. We studied the growth effects in rats of a complex solid fiber-based diet (Prolab Rodent Diet) with that of three isocaloric and isonitrogenous commercially available liquid feeding formulas which differ in their type of protein (Osmolite HN, an intact protein formula; Reabilan HN, a peptide formula; and Vivonex-TEN, an amino acid formula). Total body weight gain was greatest with the rodent diet (93 ± 2 g/10 days), followed by the peptide diet (72 ± 5 g/10 days) and intact protein diet (58 ± 8 g/10 days). Weight gain was significantly lower on the amino acid diet (43 ± 7 g/10 days). Proximal and mid gut mass was comparable with all four diets, but distal gut mass was significantly lower with the amino acid diet. Somatomedin C levels on the rodent diet (13.3 ± 1.8 nM), and the peptide diet (14.0 ± 3.3 nM) were significantly higher than somatomedin C levels on the amino acid diet (8.0 ± 1.0 nM). Somatomedin C levels on the intact protein diet (9.6 ± 1.4 nM) were intermediate between the rodent diet and amino acid diet. We conclude that growth effects vary with different enteral diets (unrelated to total calories and protein) and may result from differences in the generation of tissue growth factors.