The plasma kallikrein-kinin system, a potent vasodilator, has been implicated in causing the protein loss of the idiopathic nephrotic syndrome. However, the kidney possesses a kallikrein-kinin system separate from the plasma system. Thus, urinary kallikrein may reflect more accurately intrarenal events. Using a radiochemical esterolytic assay we measured the urinary kallikrein excretion in a patient with a minimal lesion nephrotic syndrome during relapse. Protein excretion was initially elevated (8.1 plus or minus 2.0 gm. per 24 hours) as was urinary kallikrein excretion (96.4 plus or minus 46.6 EU per 24 hours). After initiation of steroid therapy protein and kallikrein excretion decreased significantly (p less than 0.05). During the entire study kallikrein excretion was significantly correlated with protein excretion (r equals 0.89, p less than 0.01). It is tempting to speculate that activation of the intrarenal kallikrein-kinin system participates in the protein loss characteristic of the nephrotic syndrome.