The renal glomerular and tubular transport rate of amylase was studied by measuring the urinary excretion of this protein before and during inhibition of tubular protein reabsorption by lysine. The excretion of amylase was compared with the excretion of albumin, beta-2 microglobulin and free light chains of immunoglobulins. This investigation showed that amylase is reabsorbed by the tubular cells, but only to a very modest degree compared with the reabsorption of the other three proteins. In the case of amylase only about 45% of the filtered molecules are reabsorbed, whereas more than 90% of the filtered amount of the other molecules is reabsorbed by the tubular cells. The excretion of amylase rose after lysine injection only by a factor 1.8, whereas excretion rose by a factor 28 for albumin, 1,500 for beta-2 microglobulin, 16 for kappa chains and 8 for lambda chains. Minimal values for tubular reabsorption were found to be 5.5 +/- (SD) 4.3 U/min for amylase, 174.0 +/- 35.7 micrograms/min for albumin, 90.5 +/- 14.4 micrograms/min for beta-2-microglobulin, 70.4 +/- 17.4 micrograms/min for kappa chains and 24.2 +/- 9.2 micrograms/min for lambda chains.