1. The effect of oral magnesium supplementation was assessed in a group of untreated, mild hypertensive subjects who were following a moderately low sodium diet (baseline urinary sodium 86 ± 6 mmol/day).
2. Thirteen patients were allocated placebo and 12 were allocated magnesium aspartate (10 mmol/day) in a double blind fashion for 8 weeks.
3. There was no fall in blood pressure with magnesium supplementation and no significant difference in blood pressure between groups at the end of the study.
4. In the magnesium group there was an increase in urinary magnesium (4.4 ± 0.6 to 6.1 ± 1 mmol/day P < 0.001), and urinary calcium (4.3 ± 0.7 to 5.5 ± 0.9 mmol/day P < 0.01). There was no change in plasma or red cell magnesium or plasma renin activity with supplementation.
5. Magnesium supplementation does not appear to have an additive hypotensive effect in mild hypertensive subjects on a reduced sodium intake.