Previous studies have assessed the protective effect of nebulized magnesium sulphate on bronchial hyperreactivity. This study investigated the effect of histamine challenge on intracellular (erythrocytes) and extracellular (plasma) levels of magnesium and the possible relationship between degree of bronchial hyperreactivity and levels of Mg in plasma and erythrocytes. The authors studied 42 mildly asthmatic patients (10 on inhaled steroids) and 20 healthy subjects. Histamine challenge was performed by the dosimeter method and provocative dose causing a 20% fall in forced expiratory volume in one second (PD20) (FEV1) was calculated. Mg levels were measured with a calmagite colourimetric assay, both at baseline and when FEV1 had fallen by 20%. The results showed that Mg levels in plasma did not significantly change after histamine challenge (from 2.06+/-0.02 mg x dL(-1) to 2.08+/-0.02 mg x dL(-1) respectively, p=0.14). Conversely there was a statistically significant decrease in Mg levels in erythrocytes between these two time points (from 1.84+/-0.02 fmmol x cell to 1.78+/-0.02 fmmol x cell p<0.0001). Similar results were observed when the subgroups were studied separately. There was no significant correlation between PD20, the difference in both magnesium concentrations (baseline-PD20 time) or the initial values of Mg levels in erythrocytes and plasma. To conclude, histamine challenge reduces magnesium levels in erythrocytes while plasma levels remain unchanged. This histamine-induced decrease in magnesium levels occurs regardless of the diagnosis of asthma, and it is not correlated with the degree of bronchial hyperreactivity.