A 13-week subchronic toxicity study of L-histidine monohydrochloride was performed in male and female F344 rats at dose levels of 0, 0.31, 0.62, 1.25, 2.5 and 5.0% in the diet, to determine the maximum tolerable dose (MTD) for subsequent investigation of carcinogenicity. Rats were randomly allocated to 6 groups, each consisting of 10 males and 10 females. No animals died during the administration period. Suppression of body weight gain and decrease in food consumption were observed in males of the 5.0% group along with hematological examination as revealed by increases in hemoglobin volume and hematocrit. Serum biochemical examination demonstrated increased levels of BUN and creatinine in females of the 5.0% group, and increased level of BUN in females of the 1.25% group. Histopathologically, sperm granulomas in the epididymis were found in half of the 5.0% group males. Based on the results of the present study, it was concluded that the MTD of L-histidine monohydrochloride is 2.5% in diet, because the dietary dose level of 5.0% proved to exert significant toxicity reflected in suppression of body weight gain and formation of sperm granulomas.