Antiluminal and luminal transport of amino acids has been studied in vivo with a microinjection technique in maleate-treated rats. Upon microinjections of l -leucine-<sup>3</sup>H and inulin-<sup>14</sup>C in the lumen of proximal convoluted tubules and in peritubular capillaries of the cortical segments of the nephron, the amount of radioactivity measured in ureteral urine gave an indication of amino acid absorption. The results suggest that the entry of amino acids into the renal tubule cells is not impeded in maleate-treated rats and that maleic acid does not act as a competitor for amino acid transport. After microinjections into peritubular capillaries of the left kidney, L-leucine excretion was found to be greater in this experimental kidney than in the contralateral non-injected kidney. However, in maleate-treated rats, the excretion of leucine preceded that of inulin in the experimental kidney in contrast to the non-injected kidney where leucine and inulin excretion occurred simultaneously. It is proposed that this modification of amino acid excretion can be explained by an enhanced transtubular flux of amino acid, caused by a decreased ability of the cell to retain absorbed amino acids.