Oral methionine supplements (8.0 g daily for 4 days) were given to five normal volunteers who continued to eat their usual diet. This treatment resulted in a significant fall in serum folate concentration. Three days after the end of treatment concentrations had not completely returned to control values. The fall in concentration was prevented by giving oral folic acid supplements. It is suggested that folic acid supplements should be given to patients who are receiving intravenous infusions of amino acid mixtures.