1. Urinary kallikrein excretion was measured in rats by an enzyme kinetic method employing radioimmunoassay of generated bradykinin.
2. Rats given a sodium load (NaCl solution, 20 g/l, to drink) for 28 days showed acute and prolonged significant falls in urinary kallikrein excretion associated with suppression of plasma renin and angiotensin.
3. Conversely sodium-depleted rats showed increases in urinary kallikrein excretion, associated with rises in plasma renin and angiotensin.
4. A close and significant direct relation between plasma renin activity and urinary kallikrein excretion was demonstrated.
5. The diuresis and natriuresis induced by frusemide in rats was associated with increased urinary kallikrein excretion and acute rises in plasma renin.
6. In chronic renal hypertensive rats urinary kallikrein excretion was increased only in the animals with two-kidney Goldblatt hypertension. This group was also the only group that demonstrated a significant rise in plasma renin activity.