The purpose of this 85-day study was to investigate the long-term effects of histidine depletion on nitrogen utilization in young adult men. A low nitrogen (6.3 g/day), low histidine (10 mg/day) amino acid diet was fed to seven men for 8 weeks. Mean nitrogen balance became negative at the end of the 8-week period. Free histidine in postabsorptive plasma and 24-hour urine decreased significantly during the first 2 weeks of the depletion and remained low and constant for the remaining 6 weeks. Hemoglobin concentration decreased somewhat, and serum iron concentration increased significantly during histidine depletion. Lean body mass, urinary N'-methylhistidine and total creatinine did not change significantly. On addition of histidine to the low histidine diet for 2 weeks, nitrogen retention became positive, plasma and urinary histidine returned to initial values, serum iron fell, and hemoglobin concentration slowly increased. These parameters remained unchanged in two control men fed the same diet supplemented with histidine (1.05 g/day) for 8 weeks. The results suggest that histidine is indispensable for young men consuming a low nitrogen diet.