The results showed a decrease of less than 2% in the tubular reabsorption of 1 ( - )-tryptophane, 1(+)-isoleucine, 1( + )-valine and 1(+)-leucine even though their plasma concns. were increased as much as 10-70-fold. The variations in clearance for the compounds at normal and maximal blood levels were: l(-)-tryptophane, 0.18 to 0.44 ml./min., blood level 13.5-168 [gamma]/ml.; 1 (+) -leucine, 0.10-0.13 ml./min., blood level 26.0-265 [gamma]/ml.; dl-isoleucine 0.07-1.17 ml./min., blood 14.0-960 [gamma]/ml.; dl-valine 0.0-0.27 ml./min., blood level 39.0-1046 [gamma]/ml. From these results it appears that the maximal reabsorptive capacity of the renal tubules for these amino acids could not be approximated at dosages that were feasible for study.