The relation for phosphate between the excretion rate (UV) and the plasma concentration [P] has been defined in fasting persons and after an infusion of phosphate when tubular reabsorption is maximal. The results demonstrate that the maximal tubular reabsorption capacity can be predicted with a nomogram from measurements of phosphate and creatinine in plasma and in an untimed specimen of urine, from the fasting person. This assessment (θ) of tubular reabsorption capacity was compared with the other indices of tubular reabsorption; θ has several advantages over the others.