1. Urinary kallikrein excreted by normal rats is significantly increased (P < 0·001) 2 h after: (a) water loading, (b) water loading plus frusemide, 0·27 mmol (10 mg) per rat, (c) salt loading. In water-loaded rats, 5 i.u. of renin strikingly reduced kallikrein excretion (P < 0·01) but considerably increased sodium excretion (P < 0·001).
2. Renal kallikrein, measured by its kininogenase activity within 2 h of water loading, was significantly increased (P < 0·05); after water loading and frusemide it was 40% decreased (P < 0·001) and after salt loading it was reduced by approximately 50% (P < 0·02). Renin did not change renal kallikrein.
3. Severely hypertensive (one-kidney) rats (blood pressure >150 mmHg) showed no increase of urinary kallikrein after water loading, although there was a marked natriuresis; in moderately hypertensive rats (blood pressure <150 mmHg) urinary kallikrein was only one-third of that observed in control normotensive rats, after an equal degree of water loading.