The aim of the present study was to evaluate the relationship between coronary vascular tone and serum magnesium concentration in patients with ischaemic heart disease. Thirty-seven patients were subjected to intracoronary injection of 200 micrograms nitroglycerine after discontinuation of anti-anginal medication for 24 h (group I, n = 23), except if they received calcium channel blockers (group II, n = 14). Coronary angiographic images were acquired before and 2 min after the nitroglycerine injection. Using quantitative coronary arteriography, the luminal diameter of one middle-sized non-stenotic coronary artery segment in each patient was measured before and after nitroglycerine injection, and the percentual increase was computed. In group I, coronary artery diameter changed from 3.0 +/- 0.7 to 3.5 +/- 0.6 mm (mean +/- SD), representing a 16.3 per cent increase. As expected, a more blunted response was observed in group II (from 3.5 +/- 0.8 to 3.7 +/- 1.0 mm, 6.5 per cent increase). However, neither of the groups displayed significant correlation between serum magnesium levels and the measured response to intracoronary nitroglycerine. In conclusion, this study fails to provide any correlation between serum magnesium levels within the normal physiological range and coronary vascular reactivity in patients with ischaemic heart disease.