We analyzed the relationship between functional damage and transport processes in the kidney in patients with glomerulonephritis and renal failure by a new analytic method. In renal failure patients, there was substantial diminution of maximum transport of secretion in renal tubules. This reduction affected the urinary excretion of ampicillin and cephalexin substantially because both drugs depend on active renal tubular secretion. Our results indicate that dosage adjustment based on creatinine clearance is not appropriate for patients receiving drugs requiring active tubular secretion for urinary excretion. Our data point to a need for a prolongation of the dosage interval of cephalexin to 20 times that in normal subjects, while five times is recommended by the creatinine clearance. In these patients, it is therefore suggested that a dosage adjustment method that involves both factors--glomerular and renal tubular functions--is required.