The effect of ACTH administration on urinary kallikrein excretion and its relationship to changes in plasma and urine electrolytes, renin concentration and steroids was examined in normotensive and mildly hypertensive subjects. ACTH administration produced hypokalaemia, initial urinary sodium retention, a fall in active plasma renin concentration, a transient rise in plasma aldosterone concentration and sustained rises in plasma deoxycorticosterone concentration and urinary kallikrein activity. Changes in patients with mild hypertension were similar in pattern to normotensives, but urinary kallikrein concentrations were lower. The effects of ACTH on urinary kallikrein excretion appeared to be independent of aldosterone and correlated most closely with deoxycorticosterone concentrations.