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1. Aromatic acids were extracted from brain and liver of rats with phenylketonuria-like characteristics produced by administration of phenylalanine, either alone or in combination with p-chlorophenylalanine. The metabolism of the aromatic acids in these tissues was measured by gas chromatography. 2. At 1h after an intraperitoneal injection of l-phenylalanine (1g/kg) in 23-day-old rats, the phenyl-lactate concentration was 2.2mug/g in the liver and 0.43mug/g in the brain, and the concentration of o-hydroxyphenylacetate was 0.26mug/g in the liver. 3. Phenylacetate concentrations in brain and liver were 0.26 and 0.14mug/g respectively. 4. Suckling rats produced phenyl-lactate less rapidly than weanling rats, but accumulated higher concentrations in longer-term experiments. 5. Intraperitoneal injections of phenyl-lactic acid showed that this compound could directly penetrate the blood-brain barrier, and could produce similar brain/liver ratios of phenyllactate to those found after phenylalanine injection. 6. Qualitative and quantitative similarities in urinary excretion of aromatic acids between the rats used in this study and human patients with uncontrolled phenylketonuria indicate that a patient with a circulating phenylalanine concentration of the order of those achieved in the experimental animal may have aromatic acid concentrations in brain and liver comparable with those found in the rats used in the present study.