The plasma disappearance rates of human beta2-microglobulin and (hydroxy-methyl-C14)-inulin was measured in unoperated, sham-operated, ureter-ligated and nephrectomised rats. Judging from the C14-activity curves in ureter-ligated and nephrectomised rats a substantial glomerular filtration continued after obstruction of the urine flow. Almost 90% of the injected protein had disappeared from plasma 120 min after injection into nephrectomised rats indicating a considerable extra-renal elimination and invalidating attempts to evaluate different renal mechanisms of elimination before and after ureter-ligation. Sham-operation induced an increase of the plasma disappearance of both beta2-microglobulin and inulin. The latter phenomenon may explain earlier findings of similarity between the disappearance curves of low molecular weight proteins in unoperated and ureter-ligated animals and stresses the importance of using sham-operated animals as controls.