1. Methylguanidine administered orally to normal volunteers was almost completely recovered in the urine, indicating that it is absorbed in the gastrointestinal tract and is not converted into other compounds. In normal persons at least, its urinary output therefore corresponds to its metabolic production rate plus the amount ingested. 2. In normal persons, diets based on foods not containing methylguanidine (e.g. vegetarian, protein-free and milk-egg diets) caused a fall in the urinary output of methylguanidine as compared with the output of the same subjects on a free diet. Conversely, higher amounts of methylguanidine were excreted on a diet rich in broth and in boiled beef, which contain large amounts of methylguanidine formed from the oxidation of creatinine, caused by boiling. 3. Oral administration of creatinine to normal volunteers induced an immediate and marked increase in urinary excretion of methylguanidine, and the ingestion of [ methyl - 14 C]creatinine by uraemic patients was followed by the urinary excretion of labelled methylguanidine. These findings indicate that creatinine is partly converted into methylguanidine in both normal and uraemic subjects and accounts for the high metabolic production of methylguanidine in patients with renal failure, in whom the body pool of creatinine is high. 4. Creatinine, incubated at 38°C for 24 h in Krebs bicarbonate solution (pH 7.38) through which was bubbled oxygen with 15% carbon dioxide, was partially oxidized to methylguanidine. This raises the possibility that even in vivo such a conversion may occur ‘non-enzymatically’.