THE subject of this paper is amino acid transport in human kidney. My interest in this topic began with a study of familial hyperprolinemia1 which Irwin Schafer and I carried out with Mary Efron. Mary then pursued her own interests in the imino acids, and published new and important information on their metabolism in man before she died.2-4 I chose to investigate membrane transport of amino acids because it seemed an interesting challenge, initiated as it was in the supercharged environment which surrounded anything with which Mary was involved. This paper reviews briefly what we are learning about transport of free amino acids from patients with hereditary aminoacidopathies. Dent5 once said that the two-dimensional partition chromatogram of amino acids in normal human urine became so familiar to the habitual observer that deviation from the normal pattern was instantly recognized. So it was with the first chromatogram which