The aim of the study was to evaluate whether different blood sampling techniques (venous vs. capillary) influence the level of ionized magnesium. A total of 12 men and 20 women were included. A venous blood sample was collected from the antecubital vein without stasis in syringes containing 50 IU/ml electrolyte balanced heparin. Capillary tubes containing sodium heparin 50 IU/ml were used for blood sampling (150 microl) from the earlobe. A statistically significant difference between venous and capillary ionized magnesium at actual pH (0.57+/-0.04 vs. 0.55+/-0.04 mmol/L, p<0.05) was observed, whereas no difference was seen between adjusted ionized magnesium. No difference was observed between venous and capillary ionized calcium at actual pH or adjusted to pH 7.4. The pH level was significantly higher in capillary blood (7.44+/-0.02 vs. 7.48+/-0.02. p<0.00001). The higher pH in capillary blood may be due to greater exposure to air in capillary tubes, and this most likely explains the observed difference between venous and capillary ionized magnesium at actual pH.