We report the CSF and plasma amino acid concentrations and their ratios in a male patient with arginase1 deficiency with an unusual early presentation at 34 days of age. He developed hyperammonaemic coma (ammonia >400 mumol/L; normal <90 mumol/L) on postnatal day 35. CSF and plasma concentrations were assayed by ion-exchange chromatography on day 36. Arginine was increased both in plasma (971 mumol/L; controls (mean +/- 2SD) 50 +/- 42) and in CSF (157 mumol/L; controls 19 +/- 8.6), resulting in a normal CSF/plasma ratio of 0.16 (controls 0.41 +/- 0.26). Interestingly, glutamine was disproportionately high in CSF (3114 mumol/L; controls 470 +/- 236) but normal in plasma (420 mumol/L; controls 627 +/- 246); the ratio exceeded unity (7.4; controls 0.76 +/- 0.31). The CSF/plasma ratios of most neutral amino acids were elevated but not those of the imino- and of the dibasic amino acids lysine and ornithine. The mechanism leading to the increase of most neutral amino acids in brain is not known. Conclusion: A normal glutamine in plasma does not exclude an increased concentration in CSF; it could be useful to ascertain by MRS that a high CSF glutamine concentration truly reflects a high concentration in brain tissue for better understanding its pathogenesis.