Objective:The relationships between blood pressure and the urinary excretion rates of sodium and potassium during the day and at night were investigated. Methods: A total of 160 participants (135 normotensive subjects and 25 untreated patients with essential hypertension) were examined using ambulatory blood pressure monitoring and timed urine collections during waking and sleeping hours.Results:Blood pressure averaged 126/79 mmHg during waking hours and 107/62 mmHg during sleep. More sodium, potassium and aldosterone were excreted during the daytime, but the natriuretic substance kallikrein was excreted at a fixed rate throughout the 24 h. During waking hours there was poor correlation between blood pressure and urinary sodium and potassium excretion. By contrast, at night when the aldosterone: kallikrein ratio fell, the sodium and potassium excretion rates were positively correlated with blood pressureConclusions:Pressure natriuresis, not apparent during waking hours, may be unmasked at night when the balance between sodium-retaining and sodium-losing mechanisms favours natriuresis. Thus, the relationship between blood pressure and 24-h sodium excretion, usually considered to show the influence of salt intake on blood pressure, may also reflect pressure-induced natriuresis, if urine is more completely collected at night than during the day, and in circumstances favouring sodium retention during the day and sodium loss during sleep