Maximal rate of renal tubular reabsorption of glucose (TmG) was determined in a 9-month-old girl with congenital intestinal malabsorption of glucose and galactose associated with constant glucosuria. At the time of the procedure, the patient had attained normal growth and development and was enjoying good health following a 5-month dietary regimen which excluded virtually all carbohydrates other than fructose. The standard glucose titration of renal tubules was carried out, using 4 arterial plasma glucose concentrations (PG), each maintained over 3 consecutive 20-minute periods while the glomerular filtration rate was kept relatively constant. The following observations were made: 1. The rate of filtered glucose (FG) was found to exceed that of reabsorption (TG) at all PGlevels ranging from 31 to 243 mg%, revealing a very low renal plasma threshold. 2. A TmGof 76.7 mg/ min/1.73 m2was achieved at a PGlevel of 174 mg%, when FG/TG= 1.35. This amounts to 25% of the normal adult value, taken as 303 mg/min/1.73 m2. Compared with data on premature and full-term newborns, it reflects a profound deficiency of tubular reabsorption of glucose in our patient. 3. A marked drop of TGwas recorded during the final hour of titration with prolonged elevation of FG; and the observation was felt to be valid. A search of the literature revealed that others have observed the same phenomenon in subjects with suboptimal tubular reabsorption of glucose relative to the filtered load.