The effect of L-DOPA treatment on the absorption, distribution, and turnover rate of L-phenylalanine have been studied in 9 patients with parkinsonism. L-DOPA, when given with L-phenylalanine, reduced the absorption rate of this amino acid. This effect was only seen when L-DOPA and L-phenylalanine were given simultaneously. L-DOPA did not affect the plasma disappearance curve of intravenously injected L-phenylalanine. The theoretical distribution volume, the distribution rate, and turnover rate of L-phenylalanine were unaffected by the L-DOPA treatment.