AbstractWe hypothesized that alteration of the dietary arginine‐methionine balance might inhibit tumor growth and suggest nutritional strategies for cancer therapy. The Morris hepatoma 3924A was subcutaneously transplanted in ACI rats. Control diets containing normal levels of arginine, methionine, and other amino acids in replacement of protein (24%), carbohydrates (59%), fat (10%), and fiber, vitamins, and minerals (7%) were fed for 28 days. Six experimental diets were adjusted to maintain amino acids at 23–25% and carbohydrates at 58–60% these diets were 1%‐2% deficient in arginine or supplemented with 1–2% arginine (expressed as percent amino acid content of diet) in combination with normal, deficient, and supplementary levels of methionine. Daily food intake was unaffected by the experimental diets. The control groups gained 26.4±2.8 g body weight, and small body weight decrements ranged from 3.5% to 8.4% in the groups fed the experimental diets. Tumor weight of controls was 8.5±1.5% of body weight. The experimental diets that produced significant tumor growth inhibition (TGI) were 1) the arginine‐methionine‐deficient diet, 2) the arginine‐excess‐methionine‐deficient diet, 3) the arginine‐deficient diet, and 4) the excess‐arginine diet. Diets containing excess methionine failed to produce TGI. TGI resulted in tumor weights 41–46% of control values. TGI was associated with significantly lower blood urea nitrogen, plasma protein, and tumor spermidine‐to‐spermine ratio than in tumor‐bearing controls. It is concluded that dietary alteration of a single amino acid, arginine, might be a potentially useful nutritional strategy for controlling tumor growth.