Weight loss, polydipsia, polyuria, hyponatremia are symptoms often seen in patients with severe renin-induced hypertension. To investigate the role of the maturing kidney in the development of high pressure diuresis hypertension was induced in infantile (18 days old) and adult (40 days old) Sprague-Dawley rats by clipping one renal artery. In infantile rats blood pressure increase was steeper than in adult rats (7.8 vs. 3.2 mmHg/day). High pressure diuresis resulting in body weight loss was observed at systolic blood pressure levels of about 140 to 150 mmHg in infantile animals compared to 180 mmHg in adult rats. At this time fluid intake was increased to 64 in infantile and 30 ml/100 g body weight/day in adult rats. Plasma renin concentration and aldosterone were two fold higher in infantile than in adult rats. The data show that infantile rats, due to a higher activation of the renin-angiotensin-aldosterone system, have a steeper blood pressure increase and, furthermore, that during maturation of the kidney high pressure diuresis starts at lower blood pressure levels and is much more pronounced.