We proposed that increasing dietary complex carbohydrates would, by increasing fecal nitrogen loss and thus decreasing urinary nitrogen and the need for urea synthesis, ameliorate the effects of arginine deficiency. Two experiments using male, weanling, Sprague-Dawley rats with ad libitum access to water and isocaloric, isonitrogenous, 19% L-amino acid diets were carried out to test this hypothesis. The first was performed in two identical blocks of a 4 X 4 factorial experiment each with six rats per group. Four levels of arginine were fed: 0.0, 0.4, 0.8 and 1.2% of diet with four variations in dietary carbohydrates: sucrose alone, replacement of 2/3 of sucrose with dextrin, and 5 g of wheat bran or guar gum added to 100 g of the sucrose-based diets. Varying dietary carbohydrate quality did not influence growth of rats fed the 1.2% arginine diets, however, when added to the diet with 0.0 arginine, guar gum but not dextrin or wheat bran increased growth. This guar gum-induced weight increase was associated with partial reversal of arginine-deficiency-induced liver lipidosis and orotic aciduria and with higher fecal nitrogen and lower urinary nitrogen. The specificity of this guar gum effect was tested in a second experiment in which guar gum was added to diets containing 33% of the control level of arginine, histidine, leucine, methionine or phenylalanine. Only rats fed the diet deficient in arginine showed significant improvement in growth with added guar gum. We conclude that dietary guar gum but not dextrin or wheat bran, by increasing fecal nitrogen loss and decreasing the need for urea synthesis, partially ameliorates the effects of a low arginine intake. The practical application of feeding guar gum to alleviate certain ammonia-toxicity-related medical conditions is discussed.