Previous studies have assessed the protective effect of nebulized magnesium sulfate on bronchial hyperreactivity. Intravenous and inhaled magnesium sulfate have successfully been used in the treatment of acute asthma. In the present study we investigated the effect of histamine challenge on intracellular (erythrocytes) and extracellular (plasma) levels of magnesium (Mg) and the possible relationship between degree of bronchial hyperreactivity and levels of Mg in plasma and erythrocytes. We studied 42 asthmatic patients (all males, mean age 21±0.3 yrs, FEV1 91±2% pred, 10 on inhaled steroids) and 20 healthy subjects (all male, mean age 22±0.4 yrs, FEV1 93±1% pred). Histamine challenge was performed using Bronchoscreen Jaeger 6021. PD20 was calculated at the time when a 20% fall of baseline FEV1 was observed. Mg levels in plasma and erythrocytes were measured both at baseline and when the PD20 had occurred. Our results showed that magnesium levels in plasma did not significantly change after histamine challenge (from 2.06±0.02 mg/dl to 2.08±0.02 mg/dl respectively). On the other hand there was a statistically significant difference in magnesium levels in erythrocytes (from 5.16±0.07 mg/dl to 4.9±0.07 mg/dl p<0.0001). There was no correlation between PD20, the difference of both magnesium concentrations (baseline-PD20 time) as well as the initial values of magnesium levels in erythrocytes and serum. In conclusion, these data show that histamine challenge significantly affects the magnesium levels in erythrocytes while at the same time serum levels remain unchangeable. This statistical difference is not correlated with the degree of bronchial hyperreactivity (PD20). Pneumon 1999, 12 (3): 182-188