1. The effects of increasing oral doses of caffeine (45, 90, 180 and 360 mg) on effective renal plasma flow (ERPF), plasma renin activity (PRA), serum electrolytes, plasma noradrenaline, blood pressure and heart rate were studied in eight healthy male volunteers.

2. Urine volume was increased by 360 mg of caffeine only. At caffeine doses greater than 90 mg urinary sodium excretion was significantly increased. There were no changes in ERPF.

3. Serum potassium was significantly reduced by 360 mg of caffeine.

4. Caffeine increased systolic pressure in a dose related manner. Diastolic pressure was also increased, but not in relation to dose. A 360 mg dose of caffeine produced a late increase in heart rate. These changes were not associated with any alterations in PRA or in plasma noradrenaline.

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