In vivo proton magnetic resonance spectroscopy was used to investigate intracerebral phenylalanine (Phe) concentrations in nine patients with classical phenylketonuria (PKU). The study included serial examinations (n=31; plasma Phe levels: 0.47–2.24 mmol/l) of patients either receiving a Phe-restricted diet (200 mg Phe per day; four patients) or a diet rich in Phe (1000 mg Phe per day; three patients). No spectrum showed metabolic abnormalities besides elevated Phe. Difference spectroscopy yielded intracerebral Phe concentrations between 0.20 and 0.76 mmol/l. Regional variations between parieto-occipital periventricular brain, frontal brain, and cerebellum were not statistically significant. Data could be fitted assuming saturable Phe transport into the brain, based on a symmetric Michaelis–Menten model (characterized by an apparent Michaelis transport constant, K_t,app, and a maximum transport velocity, T_max) and constant Phe consumption in the brain cells (described by a velocity V_max). Non-linear least-squares fitting of the combined data from all patients yielded K_t,app=0.16±0.11 mmol/l and (T_max/V_max)=9.0±4.1. Carrier saturation and competitive inhibition of the influx of other large neutral amino acids can be expected at plasma Phe levels usually found in PKU patients.