Cerebral concentrations of phenylalanine (PHE) were measured by means of quantitative in vivo 1 H MR spectroscopy in 8 adult patients treated early for phenylketonuria type I. A 1.5–Tesla routine magnetic resonance scanner, localization sequence with short echo time (20 ms), and a fully automated data processing scheme were used. Baseline plasma PHE concentrations were 1.04 (0.70–1.39) mmol/L PHE with concurrent brain PHE concentrations of 0.27 (0.13–0.41) mmol/kg of wet weight resulting in a plasma/brain ratio of 4.12. Plasma and brain concentrations correlated significantly (Kendall T b= 0.91, p< 0.01). During an oral load with a single dose of 100 mg L-PHE per kg of body weight in four patients, plasma levels steeply increased. Concurrent brain PHE increase was less steep, was significantly delayed, and still continued up to 20 h postload. Despite the proven rise in plasma and brain concentrations of PHE, neuropsychologic examinations revealed no impairment of attentional and fine motor abilities from preload up to 20 h postload.